diff --git a/json_annotation/CHANGELOG.md b/json_annotation/CHANGELOG.md
index 99086f891..14c1969d9 100644
--- a/json_annotation/CHANGELOG.md
+++ b/json_annotation/CHANGELOG.md
@@ -1,7 +1,8 @@
-## 4.0.1-dev
+## 4.0.1
 
 - Fix a potential error with `checked: true` when `ArgumentError.message` is 
   `null`.
+- Updated `JsonSerializable.fromJson` to handle `null` values.
 
 ## 4.0.0
 
diff --git a/json_annotation/lib/src/json_serializable.g.dart b/json_annotation/lib/src/json_serializable.g.dart
index 4aa268cb6..7606a71ff 100644
--- a/json_annotation/lib/src/json_serializable.g.dart
+++ b/json_annotation/lib/src/json_serializable.g.dart
@@ -18,24 +18,25 @@ JsonSerializable _$JsonSerializableFromJson(Map<String, dynamic> json) {
       'field_rename',
       'generic_argument_factories',
       'ignore_unannotated',
-      'include_if_null',
+      'include_if_null'
     ]);
     final val = JsonSerializable(
-      anyMap: $checkedConvert(json, 'any_map', (v) => v as bool),
-      checked: $checkedConvert(json, 'checked', (v) => v as bool),
-      createFactory: $checkedConvert(json, 'create_factory', (v) => v as bool),
-      createToJson: $checkedConvert(json, 'create_to_json', (v) => v as bool),
-      disallowUnrecognizedKeys:
-          $checkedConvert(json, 'disallow_unrecognized_keys', (v) => v as bool),
+      anyMap: $checkedConvert(json, 'any_map', (v) => v as bool?),
+      checked: $checkedConvert(json, 'checked', (v) => v as bool?),
+      createFactory: $checkedConvert(json, 'create_factory', (v) => v as bool?),
+      createToJson: $checkedConvert(json, 'create_to_json', (v) => v as bool?),
+      disallowUnrecognizedKeys: $checkedConvert(
+          json, 'disallow_unrecognized_keys', (v) => v as bool?),
       explicitToJson:
-          $checkedConvert(json, 'explicit_to_json', (v) => v as bool),
+          $checkedConvert(json, 'explicit_to_json', (v) => v as bool?),
       fieldRename: $checkedConvert(json, 'field_rename',
           (v) => _$enumDecodeNullable(_$FieldRenameEnumMap, v)),
       ignoreUnannotated:
-          $checkedConvert(json, 'ignore_unannotated', (v) => v as bool),
-      includeIfNull: $checkedConvert(json, 'include_if_null', (v) => v as bool),
-      genericArgumentFactories:
-          $checkedConvert(json, 'generic_argument_factories', (v) => v as bool),
+          $checkedConvert(json, 'ignore_unannotated', (v) => v as bool?),
+      includeIfNull:
+          $checkedConvert(json, 'include_if_null', (v) => v as bool?),
+      genericArgumentFactories: $checkedConvert(
+          json, 'generic_argument_factories', (v) => v as bool?),
     );
     return val;
   }, fieldKeyMap: const {
@@ -65,37 +66,41 @@ Map<String, dynamic> _$JsonSerializableToJson(JsonSerializable instance) =>
       'include_if_null': instance.includeIfNull,
     };
 
-T _$enumDecode<T>(
-  Map<T, dynamic> enumValues,
-  dynamic source, {
-  T? unknownValue,
+K _$enumDecode<K, V>(
+  Map<K, V> enumValues,
+  Object? source, {
+  K? unknownValue,
 }) {
   if (source == null) {
-    throw ArgumentError('A value must be provided. Supported values: '
-        '${enumValues.values.join(', ')}');
+    throw ArgumentError(
+      'A value must be provided. Supported values: '
+      '${enumValues.values.join(', ')}',
+    );
   }
 
-  final value = enumValues.entries
-      .cast<MapEntry<T, String>?>()
-      .singleWhere((e) => e!.value == source, orElse: () => null)
-      ?.key;
-
-  if (value == null && unknownValue == null) {
-    throw ArgumentError('`$source` is not one of the supported values: '
-        '${enumValues.values.join(', ')}');
-  }
-  return value ?? unknownValue!;
+  return enumValues.entries.singleWhere(
+    (e) => e.value == source,
+    orElse: () {
+      if (unknownValue == null) {
+        throw ArgumentError(
+          '`$source` is not one of the supported values: '
+          '${enumValues.values.join(', ')}',
+        );
+      }
+      return MapEntry(unknownValue, enumValues.values.first);
+    },
+  ).key;
 }
 
-T? _$enumDecodeNullable<T>(
-  Map<T, dynamic> enumValues,
+K? _$enumDecodeNullable<K, V>(
+  Map<K, V> enumValues,
   dynamic source, {
-  T? unknownValue,
+  K? unknownValue,
 }) {
   if (source == null) {
     return null;
   }
-  return _$enumDecode<T>(enumValues, source, unknownValue: unknownValue);
+  return _$enumDecode<K, V>(enumValues, source, unknownValue: unknownValue);
 }
 
 const _$FieldRenameEnumMap = {
diff --git a/json_annotation/pubspec.yaml b/json_annotation/pubspec.yaml
index 7879496a9..62e04e7ba 100644
--- a/json_annotation/pubspec.yaml
+++ b/json_annotation/pubspec.yaml
@@ -1,5 +1,5 @@
 name: json_annotation
-version: 4.0.1-dev
+version: 4.0.1
 description: >-
   Classes and helper functions that support JSON code generation via the
   `json_serializable` package.
@@ -10,4 +10,4 @@ environment:
 # When changing JsonSerializable class.
 # dev_dependencies:
 #   build_runner: ^1.0.0
-#   json_serializable: ^3.1.0
+#   json_serializable: ^4.0.0
diff --git a/json_serializable/CHANGELOG.md b/json_serializable/CHANGELOG.md
index 0a4595685..bcc29a002 100644
--- a/json_serializable/CHANGELOG.md
+++ b/json_serializable/CHANGELOG.md
@@ -1,3 +1,7 @@
+## 5.0.0-dev
+
+- Implementation is now null-safe.
+
 ## 4.0.3
 
 - Correctly handle nullable values with `genericArgumentFactories`.
diff --git a/json_serializable/example/example.dart b/json_serializable/example/example.dart
index 025ae5459..28d252925 100644
--- a/json_serializable/example/example.dart
+++ b/json_serializable/example/example.dart
@@ -2,8 +2,6 @@
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
-// @dart=2.12
-
 import 'package:json_annotation/json_annotation.dart';
 
 part 'example.g.dart';
diff --git a/json_serializable/example/example.g.dart b/json_serializable/example/example.g.dart
index b5d4ad39e..a01e08473 100644
--- a/json_serializable/example/example.g.dart
+++ b/json_serializable/example/example.g.dart
@@ -1,5 +1,4 @@
 // GENERATED CODE - DO NOT MODIFY BY HAND
-// @dart=2.12
 
 part of 'example.dart';
 
diff --git a/json_serializable/lib/builder.dart b/json_serializable/lib/builder.dart
index cd9014e26..c66e3df21 100644
--- a/json_serializable/lib/builder.dart
+++ b/json_serializable/lib/builder.dart
@@ -34,7 +34,7 @@ Builder jsonSerializable(BuilderOptions options) {
       lines.add('There is a problem with "${e.key}".');
     }
     if (e.message != null) {
-      lines.add(e.message);
+      lines.add(e.message!);
     } else if (e.innerError != null) {
       lines.add(e.innerError.toString());
     }
diff --git a/json_serializable/lib/src/decode_helper.dart b/json_serializable/lib/src/decode_helper.dart
index 12002a865..f6bed5954 100644
--- a/json_serializable/lib/src/decode_helper.dart
+++ b/json_serializable/lib/src/decode_helper.dart
@@ -25,15 +25,15 @@ abstract class DecodeHelper implements HelperCore {
     Map<String, FieldElement> accessibleFields,
     Map<String, String> unavailableReasons,
   ) {
-    assert(config.createFactory);
+    assert(config.createFactory!);
     final buffer = StringBuffer();
 
-    final mapType = config.anyMap ? 'Map' : 'Map<String, dynamic>';
+    final mapType = config.anyMap! ? 'Map' : 'Map<String, dynamic>';
     buffer.write('$targetClassReference '
         '${prefix}FromJson${genericClassArgumentsImpl(true)}'
         '($mapType json');
 
-    if (config.genericArgumentFactories) {
+    if (config.genericArgumentFactories!) {
       for (var arg in element.typeParameters) {
         final helperName = fromJsonForType(
           arg.instantiate(nullabilitySuffix: NullabilitySuffix.none),
@@ -49,8 +49,8 @@ abstract class DecodeHelper implements HelperCore {
     buffer.write(') {\n');
 
     String deserializeFun(String paramOrFieldName,
-            {ParameterElement ctorParam}) =>
-        _deserializeForField(accessibleFields[paramOrFieldName],
+            {ParameterElement? ctorParam}) =>
+        _deserializeForField(accessibleFields[paramOrFieldName]!,
             ctorParam: ctorParam);
 
     final data = _writeConstructorInvocation(
@@ -72,7 +72,7 @@ abstract class DecodeHelper implements HelperCore {
     final checks = _checkKeys(accessibleFields.values
         .where((fe) => data.usedCtorParamsAndFields.contains(fe.name)));
 
-    if (config.checked) {
+    if (config.checked!) {
       final classLiteral = escapeDartString(element.name);
 
       buffer..write('''
@@ -84,12 +84,12 @@ abstract class DecodeHelper implements HelperCore {
 
       for (final field in data.fieldsToSet) {
         buffer.writeln();
-        final safeName = safeNameAccess(accessibleFields[field]);
+        final safeName = safeNameAccess(accessibleFields[field]!);
         buffer
           ..write('''
     \$checkedConvert(json, $safeName, (v) => ''')
           ..write('val.$field = ')
-          ..write(_deserializeForField(accessibleFields[field],
+          ..write(_deserializeForField(accessibleFields[field]!,
               checkedProperty: true))
           ..write(');');
       }
@@ -98,7 +98,7 @@ abstract class DecodeHelper implements HelperCore {
   }''');
 
       final fieldKeyMap = Map.fromEntries(data.usedCtorParamsAndFields
-          .map((k) => MapEntry(k, nameAccess(accessibleFields[k])))
+          .map((k) => MapEntry(k, nameAccess(accessibleFields[k]!)))
           .where((me) => me.key != me.value));
 
       String fieldKeyMapArg;
@@ -131,14 +131,14 @@ abstract class DecodeHelper implements HelperCore {
     String constantList(Iterable<FieldElement> things) =>
         'const ${jsonLiteralAsDart(things.map(nameAccess).toList())}';
 
-    if (config.disallowUnrecognizedKeys) {
+    if (config.disallowUnrecognizedKeys!) {
       final allowKeysLiteral = constantList(accessibleFields);
 
       args.add('allowedKeys: $allowKeysLiteral');
     }
 
     final requiredKeys =
-        accessibleFields.where((fe) => jsonKeyFor(fe).required).toList();
+        accessibleFields.where((fe) => jsonKeyFor(fe).required!).toList();
     if (requiredKeys.isNotEmpty) {
       final requiredKeyLiteral = constantList(requiredKeys);
 
@@ -146,7 +146,7 @@ abstract class DecodeHelper implements HelperCore {
     }
 
     final disallowNullKeys = accessibleFields
-        .where((fe) => jsonKeyFor(fe).disallowNullValue)
+        .where((fe) => jsonKeyFor(fe).disallowNullValue!)
         .toList();
     if (disallowNullKeys.isNotEmpty) {
       final disallowNullKeyLiteral = constantList(disallowNullKeys);
@@ -163,10 +163,9 @@ abstract class DecodeHelper implements HelperCore {
 
   String _deserializeForField(
     FieldElement field, {
-    ParameterElement ctorParam,
-    bool checkedProperty,
+    ParameterElement? ctorParam,
+    bool checkedProperty = false,
   }) {
-    checkedProperty ??= false;
     final jsonKeyName = safeNameAccess(field);
     final targetType = ctorParam?.type ?? field.type;
     final contextHelper = getHelperContext(field);
@@ -174,7 +173,7 @@ abstract class DecodeHelper implements HelperCore {
 
     String value;
     try {
-      if (config.checked) {
+      if (config.checked!) {
         value = contextHelper
             .deserialize(
               targetType,
@@ -205,7 +204,7 @@ abstract class DecodeHelper implements HelperCore {
     final jsonKey = jsonKeyFor(field);
     final defaultValue = jsonKey.defaultValue;
     if (defaultValue != null) {
-      if (jsonKey.disallowNullValue && jsonKey.required) {
+      if (jsonKey.disallowNullValue! && jsonKey.required!) {
         log.warning('The `defaultValue` on field `${field.name}` will have no '
             'effect because both `disallowNullValue` and `required` are set to '
             '`true`.');
diff --git a/json_serializable/lib/src/encoder_helper.dart b/json_serializable/lib/src/encoder_helper.dart
index 6ea548781..c11773a3f 100644
--- a/json_serializable/lib/src/encoder_helper.dart
+++ b/json_serializable/lib/src/encoder_helper.dart
@@ -17,7 +17,7 @@ abstract class EncodeHelper implements HelperCore {
   String _fieldAccess(FieldElement field) => '$_toJsonParamName.${field.name}';
 
   Iterable<String> createToJson(Set<FieldElement> accessibleFields) sync* {
-    assert(config.createToJson);
+    assert(config.createToJson!);
 
     final buffer = StringBuffer();
 
@@ -25,7 +25,7 @@ abstract class EncodeHelper implements HelperCore {
     buffer.write('Map<String, dynamic> '
         '$functionName($targetClassReference $_toJsonParamName');
 
-    if (config.genericArgumentFactories) {
+    if (config.genericArgumentFactories!) {
       for (var arg in element.typeParameters) {
         final helperName = toJsonForType(
           arg.instantiate(nullabilitySuffix: NullabilitySuffix.none),
@@ -140,7 +140,7 @@ abstract class EncodeHelper implements HelperCore {
   bool _writeJsonValueNaive(FieldElement field) {
     final jsonKey = jsonKeyFor(field);
 
-    return jsonKey.includeIfNull ||
+    return jsonKey.includeIfNull! ||
         (!field.type.isNullableType && !_fieldHasCustomEncoder(field));
   }
 
diff --git a/json_serializable/lib/src/field_helpers.dart b/json_serializable/lib/src/field_helpers.dart
index 932c43df5..83b293c90 100644
--- a/json_serializable/lib/src/field_helpers.dart
+++ b/json_serializable/lib/src/field_helpers.dart
@@ -17,9 +17,9 @@ class _FieldSet implements Comparable<_FieldSet> {
   _FieldSet._(this.field, this.sortField)
       : assert(field.name == sortField.name);
 
-  factory _FieldSet(FieldElement classField, FieldElement superField) {
+  factory _FieldSet(FieldElement? classField, FieldElement? superField) {
     // At least one of these will != null, perhaps both.
-    final fields = [classField, superField].where((fe) => fe != null).toList();
+    final fields = [classField, superField].whereType<FieldElement>().toList();
 
     // Prefer the class field over the inherited field when sorting.
     final sortField = fields.first;
@@ -58,9 +58,9 @@ class _FieldSet implements Comparable<_FieldSet> {
     /// Returns the offset of given field/property in its source file – with a
     /// preference for the getter if it's defined.
     int _offsetFor(FieldElement e) {
-      if (e.getter != null && e.getter.nameOffset != e.nameOffset) {
+      if (e.getter != null && e.getter!.nameOffset != e.nameOffset) {
         assert(e.nameOffset == -1);
-        return e.getter.nameOffset;
+        return e.getter!.nameOffset;
       }
       return e.nameOffset;
     }
diff --git a/json_serializable/lib/src/generator_helper.dart b/json_serializable/lib/src/generator_helper.dart
index 011fd46d3..95c7d1e36 100644
--- a/json_serializable/lib/src/generator_helper.dart
+++ b/json_serializable/lib/src/generator_helper.dart
@@ -41,7 +41,7 @@ class GeneratorHelper extends HelperCore with EncodeHelper, DecodeHelper {
   Iterable<String> generate() sync* {
     assert(_addedMembers.isEmpty);
 
-    if (config.genericArgumentFactories && element.typeParameters.isEmpty) {
+    if (config.genericArgumentFactories! && element.typeParameters.isEmpty) {
       log.warning(
         'The class `${element.displayName}` is annotated '
         'with `JsonSerializable` field `genericArgumentFactories: true`. '
@@ -68,7 +68,7 @@ class GeneratorHelper extends HelperCore with EncodeHelper, DecodeHelper {
           unavailableReasons[field.name] =
               'Setter-only properties are not supported.';
           log.warning('Setters are ignored: ${element.name}.${field.name}');
-        } else if (jsonKeyFor(field).ignore) {
+        } else if (jsonKeyFor(field).ignore!) {
           unavailableReasons[field.name] =
               'It is assigned to an ignored field.';
         } else {
@@ -81,7 +81,7 @@ class GeneratorHelper extends HelperCore with EncodeHelper, DecodeHelper {
     );
 
     var accessibleFieldSet = accessibleFields.values.toSet();
-    if (config.createFactory) {
+    if (config.createFactory!) {
       final createResult = createFactory(accessibleFields, unavailableReasons);
       yield createResult.output;
 
@@ -108,7 +108,7 @@ class GeneratorHelper extends HelperCore with EncodeHelper, DecodeHelper {
       },
     );
 
-    if (config.createToJson) {
+    if (config.createToJson!) {
       yield* createToJson(accessibleFieldSet);
     }
 
diff --git a/json_serializable/lib/src/helper_core.dart b/json_serializable/lib/src/helper_core.dart
index 4e389f58e..20eb2fb7b 100644
--- a/json_serializable/lib/src/helper_core.dart
+++ b/json_serializable/lib/src/helper_core.dart
@@ -30,7 +30,7 @@ abstract class HelperCore {
       '${element.name}${genericClassArgumentsImpl(false)}';
 
   @protected
-  String nameAccess(FieldElement field) => jsonKeyFor(field).name;
+  String nameAccess(FieldElement field) => jsonKeyFor(field).name!;
 
   @protected
   String safeNameAccess(FieldElement field) =>
@@ -62,7 +62,7 @@ InvalidGenerationSourceError createInvalidGenerationError(
 ) {
   var message = 'Could not generate `$targetMember` code for `${field.name}`';
 
-  String todo;
+  String? todo;
   if (error.type is TypeParameterType) {
     message = '$message because of type '
         '`${error.type.getDisplayString(withNullability: false)}` (type parameter)';
@@ -76,7 +76,7 @@ $converterOrKeyInstructions
     message = '$message because of type `${typeToCode(error.type)}`';
   } else {
     todo = '''
-To support the type `${error.type.element.name}` you can:
+To support the type `${error.type.element!.name}` you can:
 $converterOrKeyInstructions''';
   }
 
@@ -112,13 +112,13 @@ $converterOrKeyInstructions''';
 /// ```
 /// "<T as num, S>"
 /// ```
-String genericClassArguments(ClassElement element, bool withConstraints) {
+String genericClassArguments(ClassElement element, bool? withConstraints) {
   if (withConstraints == null || element.typeParameters.isEmpty) {
     return '';
   }
   final values = element.typeParameters.map((t) {
     if (withConstraints && t.bound != null) {
-      final boundCode = typeToCode(t.bound);
+      final boundCode = typeToCode(t.bound!);
       return '${t.name} extends $boundCode';
     } else {
       return t.name;
diff --git a/json_serializable/lib/src/json_key_utils.dart b/json_serializable/lib/src/json_key_utils.dart
index 0a51473c4..cde0bf487 100644
--- a/json_serializable/lib/src/json_key_utils.dart
+++ b/json_serializable/lib/src/json_key_utils.dart
@@ -34,7 +34,7 @@ JsonKey _from(FieldElement element, JsonSerializable classAnnotation) {
   /// Returns a literal value for [dartObject] if possible, otherwise throws
   /// an [InvalidGenerationSourceError] using [typeInformation] to describe
   /// the unsupported type.
-  Object literalForObject(
+  Object? literalForObject(
     DartObject dartObject,
     Iterable<String> typeInformation,
   ) {
@@ -44,7 +44,7 @@ JsonKey _from(FieldElement element, JsonSerializable classAnnotation) {
 
     final reader = ConstantReader(dartObject);
 
-    String badType;
+    String? badType;
     if (reader.isSymbol) {
       badType = 'Symbol';
     } else if (reader.isType) {
@@ -53,7 +53,7 @@ JsonKey _from(FieldElement element, JsonSerializable classAnnotation) {
       // TODO: Support calling function for the default value?
       badType = 'Function';
     } else if (!reader.isLiteral) {
-      badType = dartObject.type.element.name;
+      badType = dartObject.type!.element!.name;
     }
 
     if (badType != null) {
@@ -93,8 +93,8 @@ JsonKey _from(FieldElement element, JsonSerializable classAnnotation) {
       ];
       return reader.mapValue.map(
         (k, v) => MapEntry(
-          literalForObject(k, mapTypeInformation),
-          literalForObject(v, mapTypeInformation),
+          literalForObject(k!, mapTypeInformation),
+          literalForObject(v!, mapTypeInformation),
         ),
       );
     }
@@ -114,15 +114,15 @@ JsonKey _from(FieldElement element, JsonSerializable classAnnotation) {
   /// If [mustBeEnum] is `true`, throws an [InvalidGenerationSourceError] if
   /// either the annotated field is not an `enum` or `List` or if the value in
   /// [fieldName] is not an `enum` value.
-  Object _annotationValue(String fieldName, {bool mustBeEnum = false}) {
+  Object? _annotationValue(String fieldName, {bool mustBeEnum = false}) {
     final annotationValue = obj.read(fieldName);
 
     final enumFields = annotationValue.isNull
         ? null
-        : iterateEnumFields(annotationValue.objectValue.type);
+        : iterateEnumFields(annotationValue.objectValue.type!);
     if (enumFields != null) {
       if (mustBeEnum) {
-        DartType targetEnumType;
+        late DartType targetEnumType;
         if (isEnum(element.type)) {
           targetEnumType = element.type;
         } else if (coreIterableTypeChecker.isAssignableFromType(element.type)) {
@@ -134,8 +134,7 @@ JsonKey _from(FieldElement element, JsonSerializable classAnnotation) {
             'Iterable, List, or Set instances of an enum type.',
           );
         }
-        assert(targetEnumType != null);
-        final annotatedEnumType = annotationValue.objectValue.type;
+        final annotatedEnumType = annotationValue.objectValue.type!;
         if (!_interfaceTypesEqual(annotatedEnumType, targetEnumType)) {
           throwUnsupported(
             element,
@@ -153,7 +152,7 @@ JsonKey _from(FieldElement element, JsonSerializable classAnnotation) {
       final enumValueName = enumValueForDartObject<String>(
           annotationValue.objectValue, enumValueNames, (n) => n);
 
-      return '${annotationValue.objectValue.type.element.name}.$enumValueName';
+      return '${annotationValue.objectValue.type!.element!.name}.$enumValueName';
     } else {
       final defaultValueLiteral = annotationValue.isNull
           ? null
@@ -175,11 +174,11 @@ JsonKey _from(FieldElement element, JsonSerializable classAnnotation) {
     classAnnotation,
     element,
     defaultValue: _annotationValue('defaultValue'),
-    disallowNullValue: obj.read('disallowNullValue').literalValue as bool,
-    ignore: obj.read('ignore').literalValue as bool,
-    includeIfNull: obj.read('includeIfNull').literalValue as bool,
-    name: obj.read('name').literalValue as String,
-    required: obj.read('required').literalValue as bool,
+    disallowNullValue: obj.read('disallowNullValue').literalValue as bool?,
+    ignore: obj.read('ignore').literalValue as bool?,
+    includeIfNull: obj.read('includeIfNull').literalValue as bool?,
+    name: obj.read('name').literalValue as String?,
+    required: obj.read('required').literalValue as bool?,
     unknownEnumValue: _annotationValue('unknownEnumValue', mustBeEnum: true),
   );
 }
@@ -187,13 +186,13 @@ JsonKey _from(FieldElement element, JsonSerializable classAnnotation) {
 JsonKey _populateJsonKey(
   JsonSerializable classAnnotation,
   FieldElement element, {
-  Object defaultValue,
-  bool disallowNullValue,
-  bool ignore,
-  bool includeIfNull,
-  String name,
-  bool required,
-  Object unknownEnumValue,
+  Object? defaultValue,
+  bool? disallowNullValue,
+  bool? ignore,
+  bool? includeIfNull,
+  String? name,
+  bool? required,
+  Object? unknownEnumValue,
 }) {
   if (disallowNullValue == true) {
     if (includeIfNull == true) {
@@ -218,8 +217,11 @@ JsonKey _populateJsonKey(
   return jsonKey;
 }
 
-String _encodedFieldName(JsonSerializable classAnnotation,
-    String jsonKeyNameValue, FieldElement fieldElement) {
+String _encodedFieldName(
+  JsonSerializable classAnnotation,
+  String? jsonKeyNameValue,
+  FieldElement fieldElement,
+) {
   if (jsonKeyNameValue != null) {
     return jsonKeyNameValue;
   }
@@ -233,20 +235,20 @@ String _encodedFieldName(JsonSerializable classAnnotation,
       return kebabCase(fieldElement.name);
     case FieldRename.pascal:
       return pascalCase(fieldElement.name);
+    default:
+      throw ArgumentError.value(
+        classAnnotation,
+        'classAnnotation',
+        'The provided `fieldRename` (${classAnnotation.fieldRename}) is not '
+            'supported.',
+      );
   }
-
-  throw ArgumentError.value(
-    classAnnotation,
-    'classAnnotation',
-    'The provided `fieldRename` (${classAnnotation.fieldRename}) is not '
-        'supported.',
-  );
 }
 
-bool _includeIfNull(
-  bool keyIncludeIfNull,
-  bool keyDisallowNullValue,
-  bool classIncludeIfNull,
+bool? _includeIfNull(
+  bool? keyIncludeIfNull,
+  bool? keyDisallowNullValue,
+  bool? classIncludeIfNull,
 ) {
   if (keyDisallowNullValue == true) {
     assert(keyIncludeIfNull != true);
diff --git a/json_serializable/lib/src/json_part_builder.dart b/json_serializable/lib/src/json_part_builder.dart
index 1bf2d38f3..8cc040444 100644
--- a/json_serializable/lib/src/json_part_builder.dart
+++ b/json_serializable/lib/src/json_part_builder.dart
@@ -16,8 +16,8 @@ import 'settings.dart';
 /// [formatOutput] is called to format the generated code. If not provided,
 /// the default Dart code formatter is used.
 Builder jsonPartBuilder({
-  String Function(String code) formatOutput,
-  JsonSerializable config,
+  String Function(String code)? formatOutput,
+  JsonSerializable? config,
 }) {
   final settings = Settings(config: config);
 
diff --git a/json_serializable/lib/src/json_serializable_generator.dart b/json_serializable/lib/src/json_serializable_generator.dart
index e0894c58a..0513a7716 100644
--- a/json_serializable/lib/src/json_serializable_generator.dart
+++ b/json_serializable/lib/src/json_serializable_generator.dart
@@ -30,8 +30,8 @@ class JsonSerializableGenerator
   /// [BigIntHelper], [DateTimeHelper], [DurationHelper], [JsonHelper], and
   /// [UriHelper].
   factory JsonSerializableGenerator({
-    JsonSerializable config,
-    List<TypeHelper> typeHelpers,
+    JsonSerializable? config,
+    List<TypeHelper>? typeHelpers,
   }) =>
       JsonSerializableGenerator.fromSettings(Settings(
         config: config,
@@ -46,7 +46,7 @@ class JsonSerializableGenerator
   /// [UriHelper].
   factory JsonSerializableGenerator.withDefaultHelpers(
     Iterable<TypeHelper> typeHelpers, {
-    JsonSerializable config,
+    JsonSerializable? config,
   }) =>
       JsonSerializableGenerator(
         config: config,
@@ -61,7 +61,7 @@ class JsonSerializableGenerator
     ConstantReader annotation,
     BuildStep buildStep,
   ) {
-    if (!element.library.isNonNullableByDefault) {
+    if (!element.library!.isNonNullableByDefault) {
       throw InvalidGenerationSourceError(
         'Generator cannot target libraries that have not been migrated to '
         'null-safety.',
@@ -76,8 +76,7 @@ class JsonSerializableGenerator
       );
     }
 
-    final classElement = element as ClassElement;
-    final helper = GeneratorHelper(_settings, classElement, annotation);
+    final helper = GeneratorHelper(_settings, element, annotation);
     return helper.generate();
   }
 }
diff --git a/json_serializable/lib/src/settings.dart b/json_serializable/lib/src/settings.dart
index 50b6639de..5d9e3ea40 100644
--- a/json_serializable/lib/src/settings.dart
+++ b/json_serializable/lib/src/settings.dart
@@ -52,8 +52,8 @@ class Settings {
   /// [BigIntHelper], [DateTimeHelper], [DurationHelper], [JsonHelper], and
   /// [UriHelper].
   const Settings({
-    JsonSerializable config,
-    List<TypeHelper> typeHelpers,
+    JsonSerializable? config,
+    List<TypeHelper>? typeHelpers,
   })  : _config = config ?? JsonSerializable.defaults,
         _typeHelpers = typeHelpers ?? defaultHelpers;
 
@@ -65,7 +65,7 @@ class Settings {
   /// [UriHelper].
   factory Settings.withDefaultHelpers(
     Iterable<TypeHelper> typeHelpers, {
-    JsonSerializable config,
+    JsonSerializable? config,
   }) =>
       Settings(
         config: config,
diff --git a/json_serializable/lib/src/shared_checkers.dart b/json_serializable/lib/src/shared_checkers.dart
index 0f4a6dca3..40a78cc1e 100644
--- a/json_serializable/lib/src/shared_checkers.dart
+++ b/json_serializable/lib/src/shared_checkers.dart
@@ -3,6 +3,7 @@
 // BSD-style license that can be found in the LICENSE file.
 
 import 'package:analyzer/dart/element/type.dart';
+import 'package:collection/collection.dart';
 import 'package:source_gen/source_gen.dart' show TypeChecker;
 
 import 'helper_core.dart';
@@ -28,7 +29,7 @@ DartType coreIterableGenericType(DartType type) =>
 List<DartType> typeArgumentsOf(DartType type, TypeChecker checker) {
   final implementation = _getImplementationType(type, checker) as InterfaceType;
 
-  return implementation?.typeArguments;
+  return implementation.typeArguments;
 }
 
 /// A [TypeChecker] for [String], [bool] and [num].
@@ -79,11 +80,10 @@ Iterable<DartType> typeImplementations(DartType type) sync* {
     yield* type.mixins.expand(typeImplementations);
 
     if (type.superclass != null) {
-      yield* typeImplementations(type.superclass);
+      yield* typeImplementations(type.superclass!);
     }
   }
 }
 
-DartType _getImplementationType(DartType type, TypeChecker checker) =>
-    typeImplementations(type)
-        .firstWhere(checker.isExactlyType, orElse: () => null);
+DartType? _getImplementationType(DartType type, TypeChecker checker) =>
+    typeImplementations(type).firstWhereOrNull(checker.isExactlyType);
diff --git a/json_serializable/lib/src/type_helper.dart b/json_serializable/lib/src/type_helper.dart
index af0c1167c..3d7e10f5a 100644
--- a/json_serializable/lib/src/type_helper.dart
+++ b/json_serializable/lib/src/type_helper.dart
@@ -17,11 +17,11 @@ abstract class TypeHelperContext {
 
   /// [expression] may be just the name of the field or it may an expression
   /// representing the serialization of a value.
-  Object serialize(DartType fieldType, String expression);
+  Object? serialize(DartType fieldType, String expression);
 
   /// [expression] may be just the name of the field or it may an expression
   /// representing the serialization of a value.
-  Object deserialize(DartType fieldType, String expression);
+  Object? deserialize(DartType fieldType, String expression);
 
   /// Adds [memberContent] to the set of generated, top-level members.
   void addMember(String memberContent);
@@ -51,7 +51,7 @@ abstract class TypeHelper<T extends TypeHelperContext> {
   /// String serialize(DartType targetType, String expression) =>
   ///   "$expression.id";
   /// ```.
-  Object serialize(DartType targetType, String expression, T context);
+  Object? serialize(DartType targetType, String expression, T context);
 
   /// Returns Dart code that deserializes an [expression] representing a JSON
   /// literal to into [targetType].
@@ -76,7 +76,7 @@ abstract class TypeHelper<T extends TypeHelperContext> {
   /// String deserialize(DartType targetType, String expression) =>
   ///   "new ${targetType.name}.fromInt($expression)";
   /// ```.
-  Object deserialize(
+  Object? deserialize(
     DartType targetType,
     String expression,
     T context,
diff --git a/json_serializable/lib/src/type_helper_ctx.dart b/json_serializable/lib/src/type_helper_ctx.dart
index cd1efaff5..ab1fff795 100644
--- a/json_serializable/lib/src/type_helper_ctx.dart
+++ b/json_serializable/lib/src/type_helper_ctx.dart
@@ -6,7 +6,6 @@ import 'package:analyzer/dart/constant/value.dart';
 import 'package:analyzer/dart/element/element.dart';
 import 'package:analyzer/dart/element/type.dart';
 import 'package:json_annotation/json_annotation.dart';
-import 'package:meta/meta.dart';
 
 import 'helper_core.dart';
 import 'type_helper.dart';
@@ -34,10 +33,10 @@ class TypeHelperCtx
   TypeHelperCtx._(this._helperCore, this.fieldElement);
 
   @override
-  ConvertData get serializeConvertData => _pairFromContext?.toJson;
+  ConvertData? get serializeConvertData => _pairFromContext.toJson;
 
   @override
-  ConvertData get deserializeConvertData => _pairFromContext?.fromJson;
+  ConvertData? get deserializeConvertData => _pairFromContext.fromJson;
 
   _ConvertPair get _pairFromContext => _ConvertPair(fieldElement);
 
@@ -47,17 +46,17 @@ class TypeHelperCtx
   }
 
   @override
-  Object serialize(DartType targetType, String expression) => _run(
+  Object? serialize(DartType targetType, String expression) => _run(
         targetType,
         expression,
         (TypeHelper th) => th.serialize(targetType, expression, this),
       );
 
   @override
-  Object deserialize(
+  Object? deserialize(
     DartType targetType,
     String expression, {
-    @required bool defaultProvided,
+    bool defaultProvided = false,
   }) =>
       _run(
         targetType,
@@ -66,25 +65,25 @@ class TypeHelperCtx
           targetType,
           expression,
           this,
-          defaultProvided ?? false,
+          defaultProvided,
         ),
       );
 
   Object _run(
     DartType targetType,
     String expression,
-    Object Function(TypeHelper) invoke,
+    Object? Function(TypeHelper) invoke,
   ) =>
       _helperCore.allTypeHelpers.map(invoke).firstWhere(
             (r) => r != null,
             orElse: () => throw UnsupportedTypeError(targetType, expression),
-          );
+          ) as Object;
 }
 
 class _ConvertPair {
   static final _expando = Expando<_ConvertPair>();
 
-  final ConvertData fromJson, toJson;
+  final ConvertData? fromJson, toJson;
 
   _ConvertPair._(this.fromJson, this.toJson);
 
@@ -106,7 +105,7 @@ class _ConvertPair {
   }
 }
 
-ConvertData _convertData(DartObject obj, FieldElement element, bool isFrom) {
+ConvertData? _convertData(DartObject obj, FieldElement element, bool isFrom) {
   final paramName = isFrom ? 'fromJson' : 'toJson';
   final objectValue = obj.getField(paramName);
 
@@ -114,7 +113,7 @@ ConvertData _convertData(DartObject obj, FieldElement element, bool isFrom) {
     return null;
   }
 
-  final executableElement = objectValue.toFunctionValue();
+  final executableElement = objectValue.toFunctionValue()!;
 
   if (executableElement.parameters.isEmpty ||
       executableElement.parameters.first.isNamed ||
diff --git a/json_serializable/lib/src/type_helpers/big_int_helper.dart b/json_serializable/lib/src/type_helpers/big_int_helper.dart
index 4a4166d91..b2da21a27 100644
--- a/json_serializable/lib/src/type_helpers/big_int_helper.dart
+++ b/json_serializable/lib/src/type_helpers/big_int_helper.dart
@@ -12,7 +12,7 @@ class BigIntHelper extends TypeHelper {
   const BigIntHelper();
 
   @override
-  String serialize(
+  String? serialize(
     DartType targetType,
     String expression,
     TypeHelperContext context,
@@ -24,7 +24,7 @@ class BigIntHelper extends TypeHelper {
       );
 
   @override
-  String deserialize(
+  String? deserialize(
     DartType targetType,
     String expression,
     TypeHelperContext context,
diff --git a/json_serializable/lib/src/type_helpers/convert_helper.dart b/json_serializable/lib/src/type_helpers/convert_helper.dart
index b605c5c4d..8d31f9863 100644
--- a/json_serializable/lib/src/type_helpers/convert_helper.dart
+++ b/json_serializable/lib/src/type_helpers/convert_helper.dart
@@ -18,16 +18,16 @@ class ConvertData {
 }
 
 abstract class TypeHelperContextWithConvert extends TypeHelperContext {
-  ConvertData get serializeConvertData;
+  ConvertData? get serializeConvertData;
 
-  ConvertData get deserializeConvertData;
+  ConvertData? get deserializeConvertData;
 }
 
 class ConvertHelper extends TypeHelper<TypeHelperContextWithConvert> {
   const ConvertHelper();
 
   @override
-  String serialize(
+  String? serialize(
     DartType targetType,
     String expression,
     TypeHelperContextWithConvert context,
@@ -43,7 +43,7 @@ class ConvertHelper extends TypeHelper<TypeHelperContextWithConvert> {
   }
 
   @override
-  String deserialize(
+  String? deserialize(
     DartType targetType,
     String expression,
     TypeHelperContextWithConvert context,
diff --git a/json_serializable/lib/src/type_helpers/date_time_helper.dart b/json_serializable/lib/src/type_helpers/date_time_helper.dart
index beefd0065..d97bc17ac 100644
--- a/json_serializable/lib/src/type_helpers/date_time_helper.dart
+++ b/json_serializable/lib/src/type_helpers/date_time_helper.dart
@@ -12,7 +12,7 @@ class DateTimeHelper extends TypeHelper {
   const DateTimeHelper();
 
   @override
-  String serialize(
+  String? serialize(
     DartType targetType,
     String expression,
     TypeHelperContext context,
@@ -24,7 +24,7 @@ class DateTimeHelper extends TypeHelper {
       );
 
   @override
-  String deserialize(
+  String? deserialize(
     DartType targetType,
     String expression,
     TypeHelperContext context,
diff --git a/json_serializable/lib/src/type_helpers/duration_helper.dart b/json_serializable/lib/src/type_helpers/duration_helper.dart
index 95acda21a..cf597a6be 100644
--- a/json_serializable/lib/src/type_helpers/duration_helper.dart
+++ b/json_serializable/lib/src/type_helpers/duration_helper.dart
@@ -12,7 +12,7 @@ class DurationHelper extends TypeHelper {
   const DurationHelper();
 
   @override
-  String serialize(
+  String? serialize(
     DartType targetType,
     String expression,
     TypeHelperContext context,
@@ -33,7 +33,7 @@ class DurationHelper extends TypeHelper {
   }
 
   @override
-  String deserialize(
+  String? deserialize(
     DartType targetType,
     String expression,
     TypeHelperContext context,
diff --git a/json_serializable/lib/src/type_helpers/enum_helper.dart b/json_serializable/lib/src/type_helpers/enum_helper.dart
index f12c54740..c3aa4ba31 100644
--- a/json_serializable/lib/src/type_helpers/enum_helper.dart
+++ b/json_serializable/lib/src/type_helpers/enum_helper.dart
@@ -15,7 +15,7 @@ class EnumHelper extends TypeHelper<TypeHelperContextWithConfig> {
   const EnumHelper();
 
   @override
-  String serialize(
+  String? serialize(
     DartType targetType,
     String expression,
     TypeHelperContextWithConfig context,
@@ -32,7 +32,7 @@ class EnumHelper extends TypeHelper<TypeHelperContextWithConfig> {
   }
 
   @override
-  String deserialize(
+  String? deserialize(
     DartType targetType,
     String expression,
     TypeHelperContextWithConfig context,
@@ -69,9 +69,9 @@ class EnumHelper extends TypeHelper<TypeHelperContextWithConfig> {
 }
 
 String _constMapName(DartType targetType) =>
-    '_\$${targetType.element.name}EnumMap';
+    '_\$${targetType.element!.name}EnumMap';
 
-String _enumValueMapFromType(DartType targetType) {
+String? _enumValueMapFromType(DartType targetType) {
   final enumMap = enumFieldsMap(targetType);
 
   if (enumMap == null) {
@@ -79,7 +79,7 @@ String _enumValueMapFromType(DartType targetType) {
   }
 
   final items = enumMap.entries
-      .map((e) => '  ${targetType.element.name}.${e.key.name}: '
+      .map((e) => '  ${targetType.element!.name}.${e.key.name}: '
           '${jsonLiteralAsDart(e.value)},')
       .join();
 
diff --git a/json_serializable/lib/src/type_helpers/generic_factory_helper.dart b/json_serializable/lib/src/type_helpers/generic_factory_helper.dart
index a8f8dc88c..8227065f4 100644
--- a/json_serializable/lib/src/type_helpers/generic_factory_helper.dart
+++ b/json_serializable/lib/src/type_helpers/generic_factory_helper.dart
@@ -11,12 +11,12 @@ class GenericFactoryHelper extends TypeHelper<TypeHelperContextWithConfig> {
   const GenericFactoryHelper();
 
   @override
-  Object serialize(
+  Object? serialize(
     DartType targetType,
     String expression,
     TypeHelperContextWithConfig context,
   ) {
-    if (context.config.genericArgumentFactories &&
+    if (context.config.genericArgumentFactories! &&
         targetType is TypeParameterType) {
       return LambdaResult(expression, toJsonForType(targetType));
     }
@@ -25,13 +25,13 @@ class GenericFactoryHelper extends TypeHelper<TypeHelperContextWithConfig> {
   }
 
   @override
-  Object deserialize(
+  Object? deserialize(
     DartType targetType,
     String expression,
     TypeHelperContextWithConfig context,
     bool defaultProvided,
   ) {
-    if (context.config.genericArgumentFactories &&
+    if (context.config.genericArgumentFactories! &&
         targetType is TypeParameterType) {
       return LambdaResult(expression, fromJsonForType(targetType));
     }
diff --git a/json_serializable/lib/src/type_helpers/iterable_helper.dart b/json_serializable/lib/src/type_helpers/iterable_helper.dart
index 53f2fcf69..4c26fb42e 100644
--- a/json_serializable/lib/src/type_helpers/iterable_helper.dart
+++ b/json_serializable/lib/src/type_helpers/iterable_helper.dart
@@ -15,7 +15,7 @@ class IterableHelper extends TypeHelper<TypeHelperContextWithConfig> {
   const IterableHelper();
 
   @override
-  String serialize(
+  String? serialize(
     DartType targetType,
     String expression,
     TypeHelperContextWithConfig context,
@@ -30,7 +30,7 @@ class IterableHelper extends TypeHelper<TypeHelperContextWithConfig> {
     // Although it's possible that child elements may be marked unsafe
 
     var isList = _coreListChecker.isAssignableFromType(targetType);
-    final subField = context.serialize(itemType, closureArg);
+    final subField = context.serialize(itemType, closureArg)!;
 
     var optionalQuestion = targetType.isNullableType ? '?' : '';
 
@@ -59,7 +59,7 @@ class IterableHelper extends TypeHelper<TypeHelperContextWithConfig> {
   }
 
   @override
-  String deserialize(
+  String? deserialize(
     DartType targetType,
     String expression,
     TypeHelperContext context,
@@ -73,7 +73,7 @@ class IterableHelper extends TypeHelper<TypeHelperContextWithConfig> {
 
     final iterableGenericType = coreIterableGenericType(targetType);
 
-    final itemSubVal = context.deserialize(iterableGenericType, closureArg);
+    final itemSubVal = context.deserialize(iterableGenericType, closureArg)!;
 
     var output = '$expression as List<dynamic>';
 
diff --git a/json_serializable/lib/src/type_helpers/json_converter_helper.dart b/json_serializable/lib/src/type_helpers/json_converter_helper.dart
index f1f2bdfd1..9c9044f40 100644
--- a/json_serializable/lib/src/type_helpers/json_converter_helper.dart
+++ b/json_serializable/lib/src/type_helpers/json_converter_helper.dart
@@ -5,6 +5,7 @@
 import 'package:analyzer/dart/constant/value.dart';
 import 'package:analyzer/dart/element/element.dart';
 import 'package:analyzer/dart/element/type.dart';
+import 'package:collection/collection.dart';
 import 'package:json_annotation/json_annotation.dart';
 import 'package:source_gen/source_gen.dart';
 
@@ -20,7 +21,7 @@ class JsonConverterHelper extends TypeHelper {
   const JsonConverterHelper();
 
   @override
-  Object serialize(
+  Object? serialize(
     DartType targetType,
     String expression,
     TypeHelperContext context,
@@ -35,7 +36,7 @@ class JsonConverterHelper extends TypeHelper {
   }
 
   @override
-  Object deserialize(
+  Object? deserialize(
     DartType targetType,
     String expression,
     TypeHelperContext context,
@@ -76,10 +77,10 @@ class _JsonConvertData {
       accessor.isEmpty ? '' : '.$accessor';
 }
 
-_JsonConvertData _typeConverter(DartType targetType, TypeHelperContext ctx) {
+_JsonConvertData? _typeConverter(DartType targetType, TypeHelperContext ctx) {
   List<_ConverterMatch> converterMatches(List<ElementAnnotation> items) => items
       .map((annotation) => _compatibleMatch(targetType, annotation))
-      .where((dt) => dt != null)
+      .whereType<_ConverterMatch>()
       .toList();
 
   var matchingAnnotations = converterMatches(ctx.fieldElement.metadata);
@@ -96,7 +97,7 @@ _JsonConvertData _typeConverter(DartType targetType, TypeHelperContext ctx) {
   return _typeConverterFrom(matchingAnnotations, targetType);
 }
 
-_JsonConvertData _typeConverterFrom(
+_JsonConvertData? _typeConverterFrom(
   List<_ConverterMatch> matchingAnnotations,
   DartType targetType,
 ) {
@@ -137,15 +138,15 @@ _JsonConvertData _typeConverterFrom(
 
   if (match.genericTypeArg != null) {
     return _JsonConvertData.genericClass(
-      match.annotation.type.element.name,
-      match.genericTypeArg,
+      match.annotation.type!.element!.name!,
+      match.genericTypeArg!,
       reviver.accessor,
       match.jsonType,
     );
   }
 
   return _JsonConvertData.className(
-    match.annotation.type.element.name,
+    match.annotation.type!.element!.name!,
     reviver.accessor,
     match.jsonType,
   );
@@ -155,7 +156,7 @@ class _ConverterMatch {
   final DartObject annotation;
   final DartType jsonType;
   final ElementAnnotation elementAnnotation;
-  final String genericTypeArg;
+  final String? genericTypeArg;
 
   _ConverterMatch(
     this.elementAnnotation,
@@ -165,17 +166,18 @@ class _ConverterMatch {
   );
 }
 
-_ConverterMatch _compatibleMatch(
+_ConverterMatch? _compatibleMatch(
   DartType targetType,
   ElementAnnotation annotation,
 ) {
-  final constantValue = annotation.computeConstantValue();
+  final constantValue = annotation.computeConstantValue()!;
 
-  final converterClassElement = constantValue.type.element as ClassElement;
+  final converterClassElement = constantValue.type!.element as ClassElement;
 
-  final jsonConverterSuper = converterClassElement.allSupertypes.singleWhere(
-      (e) => e is InterfaceType && _jsonConverterChecker.isExactly(e.element),
-      orElse: () => null);
+  final jsonConverterSuper =
+      converterClassElement.allSupertypes.singleWhereOrNull(
+    (e) => e is InterfaceType && _jsonConverterChecker.isExactly(e.element),
+  );
 
   if (jsonConverterSuper == null) {
     return null;
diff --git a/json_serializable/lib/src/type_helpers/json_helper.dart b/json_serializable/lib/src/type_helpers/json_helper.dart
index a678a309e..86de56fc9 100644
--- a/json_serializable/lib/src/type_helpers/json_helper.dart
+++ b/json_serializable/lib/src/type_helpers/json_helper.dart
@@ -5,6 +5,7 @@
 import 'package:analyzer/dart/element/element.dart';
 import 'package:analyzer/dart/element/nullability_suffix.dart';
 import 'package:analyzer/dart/element/type.dart';
+import 'package:collection/collection.dart';
 import 'package:json_annotation/json_annotation.dart';
 import 'package:source_gen/source_gen.dart';
 
@@ -23,7 +24,7 @@ class JsonHelper extends TypeHelper<TypeHelperContextWithConfig> {
   /// By default, JSON encoding in from `dart:convert` calls `toJson()` on
   /// provided objects.
   @override
-  String serialize(
+  String? serialize(
     DartType targetType,
     String expression,
     TypeHelperContextWithConfig context,
@@ -54,7 +55,7 @@ class JsonHelper extends TypeHelper<TypeHelperContextWithConfig> {
       );
     }
 
-    if (context.config.explicitToJson || toJsonArgs.isNotEmpty) {
+    if (context.config.explicitToJson! || toJsonArgs.isNotEmpty) {
       return '$expression${interfaceType.isNullableType ? '?' : ''}'
           '.toJson(${toJsonArgs.map((a) => '$a, ').join()} )';
     }
@@ -62,7 +63,7 @@ class JsonHelper extends TypeHelper<TypeHelperContextWithConfig> {
   }
 
   @override
-  String deserialize(
+  String? deserialize(
     DartType targetType,
     String expression,
     TypeHelperContextWithConfig context,
@@ -72,11 +73,10 @@ class JsonHelper extends TypeHelper<TypeHelperContextWithConfig> {
       return null;
     }
 
-    final type = targetType as InterfaceType;
-    final classElement = type.element;
+    final classElement = targetType.element;
 
     final fromJsonCtor = classElement.constructors
-        .singleWhere((ce) => ce.name == 'fromJson', orElse: () => null);
+        .singleWhereOrNull((ce) => ce.name == 'fromJson');
 
     var output = expression;
     if (fromJsonCtor != null) {
@@ -95,7 +95,7 @@ class JsonHelper extends TypeHelper<TypeHelperContextWithConfig> {
       var asCastType = positionalParams.first.type;
 
       if (asCastType is InterfaceType) {
-        final instantiated = _instantiate(asCastType as InterfaceType, type);
+        final instantiated = _instantiate(asCastType, targetType);
         if (instantiated != null) {
           asCastType = instantiated;
         }
@@ -108,15 +108,15 @@ class JsonHelper extends TypeHelper<TypeHelperContextWithConfig> {
         ..._helperParams(
           context.deserialize,
           _decodeHelper,
-          targetType as InterfaceType,
+          targetType,
           positionalParams.skip(1),
           fromJsonCtor,
         ),
       ];
 
       output = args.join(', ');
-    } else if (_annotation(context.config, type)?.createFactory == true) {
-      if (context.config.anyMap) {
+    } else if (_annotation(context.config, targetType)?.createFactory == true) {
+      if (context.config.anyMap!) {
         output += ' as Map';
       } else {
         output += ' as Map<String, dynamic>';
@@ -135,7 +135,7 @@ class JsonHelper extends TypeHelper<TypeHelperContextWithConfig> {
 }
 
 List<String> _helperParams(
-  Object Function(DartType, String) execute,
+  Object? Function(DartType, String) execute,
   TypeParameterType Function(ParameterElement, Element) paramMapper,
   InterfaceType type,
   Iterable<ParameterElement> positionalParams,
@@ -172,7 +172,7 @@ TypeParameterType _decodeHelper(
       type.normalParameterTypes.length == 1) {
     final funcReturnType = type.returnType;
 
-    if (param.name == fromJsonForName(funcReturnType.element.name)) {
+    if (param.name == fromJsonForName(funcReturnType.element!.name!)) {
       final funcParamType = type.normalParameterTypes.single;
 
       if ((funcParamType.isDartCoreObject && funcParamType.isNullableType) ||
@@ -203,7 +203,7 @@ TypeParameterType _encodeHelper(
       type.normalParameterTypes.length == 1) {
     final funcParamType = type.normalParameterTypes.single;
 
-    if (param.name == toJsonForName(funcParamType.element.name)) {
+    if (param.name == toJsonForName(funcParamType.element!.name!)) {
       if (funcParamType is TypeParameterType) {
         return funcParamType;
       }
@@ -238,7 +238,7 @@ bool _canSerialize(JsonSerializable config, DartType type) {
 
 /// Returns an instantiation of [ctorParamType] by providing argument types
 /// derived by matching corresponding type parameters from [classType].
-InterfaceType _instantiate(
+InterfaceType? _instantiate(
   InterfaceType ctorParamType,
   InterfaceType classType,
 ) {
@@ -260,13 +260,13 @@ InterfaceType _instantiate(
   }
 
   return ctorParamType.element.instantiate(
-    typeArguments: argTypes,
+    typeArguments: argTypes.cast<DartType>(),
     // TODO: not 100% sure nullabilitySuffix is right... Works for now
     nullabilitySuffix: NullabilitySuffix.none,
   );
 }
 
-JsonSerializable _annotation(JsonSerializable config, InterfaceType source) {
+JsonSerializable? _annotation(JsonSerializable config, InterfaceType source) {
   final annotations = const TypeChecker.fromRuntime(JsonSerializable)
       .annotationsOfExact(source.element, throwOnUnresolved: false)
       .toList();
@@ -282,6 +282,6 @@ JsonSerializable _annotation(JsonSerializable config, InterfaceType source) {
   );
 }
 
-MethodElement _toJsonMethod(DartType type) => typeImplementations(type)
+MethodElement? _toJsonMethod(DartType type) => typeImplementations(type)
     .map((dt) => dt is InterfaceType ? dt.getMethod('toJson') : null)
-    .firstWhere((me) => me != null, orElse: () => null);
+    .firstWhereOrNull((me) => me != null);
diff --git a/json_serializable/lib/src/type_helpers/map_helper.dart b/json_serializable/lib/src/type_helpers/map_helper.dart
index 5eb83300b..764cd89e9 100644
--- a/json_serializable/lib/src/type_helpers/map_helper.dart
+++ b/json_serializable/lib/src/type_helpers/map_helper.dart
@@ -3,6 +3,7 @@
 // BSD-style license that can be found in the LICENSE file.
 
 import 'package:analyzer/dart/element/type.dart';
+import 'package:collection/collection.dart';
 
 import '../constants.dart';
 import '../shared_checkers.dart';
@@ -17,7 +18,7 @@ class MapHelper extends TypeHelper<TypeHelperContextWithConfig> {
   const MapHelper();
 
   @override
-  String serialize(
+  String? serialize(
     DartType targetType,
     String expression,
     TypeHelperContextWithConfig context,
@@ -50,7 +51,7 @@ class MapHelper extends TypeHelper<TypeHelperContextWithConfig> {
   }
 
   @override
-  String deserialize(
+  String? deserialize(
     DartType targetType,
     String expression,
     TypeHelperContextWithConfig context,
@@ -76,7 +77,7 @@ class MapHelper extends TypeHelper<TypeHelperContextWithConfig> {
 
     if (!isKeyStringable) {
       if (valueArgIsAny) {
-        if (context.config.anyMap) {
+        if (context.config.anyMap!) {
           if (isLikeDynamic(keyArg)) {
             return '$expression as Map$optionalQuestion';
           }
@@ -101,7 +102,7 @@ class MapHelper extends TypeHelper<TypeHelperContextWithConfig> {
 
     final itemSubVal = context.deserialize(valueArg, closureArg);
 
-    var mapCast = context.config.anyMap ? 'as Map' : 'as Map<String, dynamic>';
+    var mapCast = context.config.anyMap! ? 'as Map' : 'as Map<String, dynamic>';
 
     if (targetTypeIsNullable) {
       mapCast += '?';
@@ -110,10 +111,10 @@ class MapHelper extends TypeHelper<TypeHelperContextWithConfig> {
     String keyUsage;
     if (isEnum(keyArg)) {
       keyUsage = context.deserialize(keyArg, _keyParam).toString();
-    } else if (context.config.anyMap &&
+    } else if (context.config.anyMap! &&
         !(keyArg.isDartCoreObject || keyArg.isDynamic)) {
       keyUsage = '$_keyParam as String';
-    } else if (context.config.anyMap &&
+    } else if (context.config.anyMap! &&
         keyArg.isDartCoreObject &&
         !keyArg.isNullableType) {
       keyUsage = '$_keyParam as Object';
@@ -123,7 +124,7 @@ class MapHelper extends TypeHelper<TypeHelperContextWithConfig> {
 
     final toFromString = _forType(keyArg);
     if (toFromString != null) {
-      keyUsage = toFromString.deserialize(keyArg, keyUsage, false, true);
+      keyUsage = toFromString.deserialize(keyArg, keyUsage, false, true)!;
     }
 
     return '($expression $mapCast)$optionalQuestion.map( '
@@ -142,8 +143,8 @@ final _instances = [
   uriString,
 ];
 
-ToFromStringHelper _forType(DartType type) =>
-    _instances.singleWhere((i) => i.matches(type), orElse: () => null);
+ToFromStringHelper? _forType(DartType type) =>
+    _instances.singleWhereOrNull((i) => i.matches(type));
 
 /// Returns `true` if [keyType] can be automatically converted to/from String –
 /// and is therefor usable as a key in a [Map].
diff --git a/json_serializable/lib/src/type_helpers/to_from_string.dart b/json_serializable/lib/src/type_helpers/to_from_string.dart
index 7a5d94715..55f8aae7e 100644
--- a/json_serializable/lib/src/type_helpers/to_from_string.dart
+++ b/json_serializable/lib/src/type_helpers/to_from_string.dart
@@ -48,7 +48,7 @@ class ToFromStringHelper {
 
   bool matches(DartType type) => _checker.isExactlyType(type);
 
-  String serialize(
+  String? serialize(
     DartType type,
     String expression,
     bool nullable,
@@ -64,7 +64,7 @@ class ToFromStringHelper {
     return '$expression.$_toString';
   }
 
-  String deserialize(
+  String? deserialize(
     DartType type,
     String expression,
     bool nullable,
diff --git a/json_serializable/lib/src/type_helpers/uri_helper.dart b/json_serializable/lib/src/type_helpers/uri_helper.dart
index 2100812a5..28e40621e 100644
--- a/json_serializable/lib/src/type_helpers/uri_helper.dart
+++ b/json_serializable/lib/src/type_helpers/uri_helper.dart
@@ -12,7 +12,7 @@ class UriHelper extends TypeHelper {
   const UriHelper();
 
   @override
-  String serialize(
+  String? serialize(
     DartType targetType,
     String expression,
     TypeHelperContext context,
@@ -24,7 +24,7 @@ class UriHelper extends TypeHelper {
       );
 
   @override
-  String deserialize(
+  String? deserialize(
     DartType targetType,
     String expression,
     TypeHelperContext context,
diff --git a/json_serializable/lib/src/type_helpers/value_helper.dart b/json_serializable/lib/src/type_helpers/value_helper.dart
index 0f9c0b43f..16719c656 100644
--- a/json_serializable/lib/src/type_helpers/value_helper.dart
+++ b/json_serializable/lib/src/type_helpers/value_helper.dart
@@ -13,7 +13,7 @@ class ValueHelper extends TypeHelper {
   const ValueHelper();
 
   @override
-  String serialize(
+  String? serialize(
     DartType targetType,
     String expression,
     TypeHelperContext context,
@@ -28,7 +28,7 @@ class ValueHelper extends TypeHelper {
   }
 
   @override
-  String deserialize(
+  String? deserialize(
     DartType targetType,
     String expression,
     TypeHelperContext context,
diff --git a/json_serializable/lib/src/unsupported_type_error.dart b/json_serializable/lib/src/unsupported_type_error.dart
index 3642cc2c4..4d9a00fb3 100644
--- a/json_serializable/lib/src/unsupported_type_error.dart
+++ b/json_serializable/lib/src/unsupported_type_error.dart
@@ -8,7 +8,7 @@ import 'package:analyzer/dart/element/type.dart';
 /// [reason].
 class UnsupportedTypeError extends Error {
   final DartType type;
-  final String reason;
+  final String? reason;
 
   /// Not currently accesses. Will likely be removed in a future release.
   final String expression;
diff --git a/json_serializable/lib/src/utils.dart b/json_serializable/lib/src/utils.dart
index 3b94f13bd..f397cdbb7 100644
--- a/json_serializable/lib/src/utils.dart
+++ b/json_serializable/lib/src/utils.dart
@@ -7,18 +7,17 @@ import 'package:analyzer/dart/element/element.dart';
 import 'package:analyzer/dart/element/nullability_suffix.dart';
 import 'package:analyzer/dart/element/type.dart';
 import 'package:json_annotation/json_annotation.dart';
-import 'package:meta/meta.dart' show alwaysThrows, required;
 import 'package:source_gen/source_gen.dart';
 
 import 'helper_core.dart';
 
 const _jsonKeyChecker = TypeChecker.fromRuntime(JsonKey);
 
-DartObject _jsonKeyAnnotation(FieldElement element) =>
+DartObject? _jsonKeyAnnotation(FieldElement element) =>
     _jsonKeyChecker.firstAnnotationOf(element) ??
     (element.getter == null
         ? null
-        : _jsonKeyChecker.firstAnnotationOf(element.getter));
+        : _jsonKeyChecker.firstAnnotationOf(element.getter!));
 
 ConstantReader jsonKeyAnnotation(FieldElement element) =>
     ConstantReader(_jsonKeyAnnotation(element));
@@ -43,7 +42,7 @@ String pascalCase(String input) {
 
 String _fixCase(String input, String separator) =>
     input.replaceAllMapped(_upperCase, (match) {
-      var lower = match.group(0).toLowerCase();
+      var lower = match.group(0)!.toLowerCase();
 
       if (match.start > 0) {
         lower = '$separator$lower';
@@ -52,13 +51,12 @@ String _fixCase(String input, String separator) =>
       return lower;
     });
 
-@alwaysThrows
-void throwUnsupported(FieldElement element, String message) =>
+Never throwUnsupported(FieldElement element, String message) =>
     throw InvalidGenerationSourceError(
         'Error with `@JsonKey` on `${element.name}`. $message',
         element: element);
 
-FieldRename _fromDartObject(ConstantReader reader) => reader.isNull
+FieldRename? _fromDartObject(ConstantReader reader) => reader.isNull
     ? null
     : enumValueForDartObject(
         reader.objectValue,
@@ -76,18 +74,18 @@ T enumValueForDartObject<T>(
 /// Return an instance of [JsonSerializable] corresponding to a the provided
 /// [reader].
 JsonSerializable _valueForAnnotation(ConstantReader reader) => JsonSerializable(
-      anyMap: reader.read('anyMap').literalValue as bool,
-      checked: reader.read('checked').literalValue as bool,
-      createFactory: reader.read('createFactory').literalValue as bool,
-      createToJson: reader.read('createToJson').literalValue as bool,
+      anyMap: reader.read('anyMap').literalValue as bool?,
+      checked: reader.read('checked').literalValue as bool?,
+      createFactory: reader.read('createFactory').literalValue as bool?,
+      createToJson: reader.read('createToJson').literalValue as bool?,
       disallowUnrecognizedKeys:
-          reader.read('disallowUnrecognizedKeys').literalValue as bool,
-      explicitToJson: reader.read('explicitToJson').literalValue as bool,
+          reader.read('disallowUnrecognizedKeys').literalValue as bool?,
+      explicitToJson: reader.read('explicitToJson').literalValue as bool?,
       fieldRename: _fromDartObject(reader.read('fieldRename')),
       genericArgumentFactories:
-          reader.read('genericArgumentFactories').literalValue as bool,
-      ignoreUnannotated: reader.read('ignoreUnannotated').literalValue as bool,
-      includeIfNull: reader.read('includeIfNull').literalValue as bool,
+          reader.read('genericArgumentFactories').literalValue as bool?,
+      ignoreUnannotated: reader.read('ignoreUnannotated').literalValue as bool?,
+      includeIfNull: reader.read('includeIfNull').literalValue as bool?,
     );
 
 /// Returns a [JsonSerializable] with values from the [JsonSerializable]
@@ -102,7 +100,7 @@ JsonSerializable _valueForAnnotation(ConstantReader reader) => JsonSerializable(
 JsonSerializable mergeConfig(
   JsonSerializable config,
   ConstantReader reader, {
-  @required ClassElement classElement,
+  required ClassElement classElement,
 }) {
   final annotation = _valueForAnnotation(reader);
 
@@ -117,7 +115,7 @@ JsonSerializable mergeConfig(
     fieldRename: annotation.fieldRename ?? config.fieldRename,
     genericArgumentFactories: annotation.genericArgumentFactories ??
         (classElement.typeParameters.isNotEmpty &&
-            config.genericArgumentFactories),
+            config.genericArgumentFactories!),
     ignoreUnannotated: annotation.ignoreUnannotated ?? config.ignoreUnannotated,
     includeIfNull: annotation.includeIfNull ?? config.includeIfNull,
   );
@@ -137,7 +135,7 @@ final _enumMapExpando = Expando<Map<FieldElement, dynamic>>();
 /// used if it's set and not `null`.
 ///
 /// If [targetType] is not an enum, `null` is returned.
-Map<FieldElement, dynamic> enumFieldsMap(DartType targetType) {
+Map<FieldElement, dynamic>? enumFieldsMap(DartType targetType) {
   MapEntry<FieldElement, dynamic> _generateEntry(FieldElement fe) {
     final annotation =
         const TypeChecker.fromRuntime(JsonValue).firstAnnotationOfExact(fe);
@@ -179,7 +177,7 @@ Map<FieldElement, dynamic> enumFieldsMap(DartType targetType) {
 /// with its values.
 ///
 /// Otherwise, `null`.
-Iterable<FieldElement> iterateEnumFields(DartType targetType) =>
+Iterable<FieldElement>? iterateEnumFields(DartType targetType) =>
     enumFieldsMap(targetType)?.keys;
 
 /// Returns a quoted String literal for [value] that can be used in generated
@@ -191,7 +189,7 @@ String escapeDartString(String value) {
   var canBeRaw = true;
 
   value = value.replaceAllMapped(_escapeRegExp, (match) {
-    final value = match[0];
+    final value = match[0]!;
     if (value == "'") {
       hasSingleQuote = true;
       return value;
@@ -267,8 +265,8 @@ String _getHexLiteral(String input) {
 extension DartTypeExtension on DartType {
   bool isAssignableTo(DartType other) =>
       // If the library is `null`, treat it like dynamic => `true`
-      element.library == null ||
-      element.library.typeSystem.isAssignableTo(this, other);
+      element!.library == null ||
+      element!.library!.typeSystem.isAssignableTo(this, other);
 }
 
 extension TypeExtension on DartType {
diff --git a/json_serializable/pubspec.yaml b/json_serializable/pubspec.yaml
index c00ac598b..d823cca4b 100644
--- a/json_serializable/pubspec.yaml
+++ b/json_serializable/pubspec.yaml
@@ -1,5 +1,5 @@
 name: json_serializable
-version: 4.0.3
+version: 5.0.0-dev
 description: >-
   Automatically generate code for converting to and from JSON by annotating
   Dart classes.
@@ -7,30 +7,34 @@ repository: https://github.com/google/json_serializable.dart/tree/master/json_se
 environment:
   # Keeping this <2.12.0 because the code is not null safe – yet!
   # https://github.com/google/json_serializable.dart/issues/821
-  sdk: '>=2.11.99 <3.0.0'
+  sdk: '>=2.12.0 <3.0.0'
 
 dependencies:
   analyzer: '>=0.41.2 <2.0.0'
   build: ^2.0.0
   build_config: ^0.4.4
+  collection: ^1.14.0
 
   # Use a tight version constraint to ensure that a constraint on
   # `json_annotation` properly constrains all features it provides.
   # For v3 only – allow a wide range since the only change was to REMOVE things
   # from json_annotation
-  json_annotation: '>=4.0.0 <4.1.0'
+  json_annotation: '>=4.0.1 <4.1.0'
   meta: ^1.3.0
   path: ^1.8.0
-  source_gen: ^0.9.9
+  source_gen: ^1.0.0
 
 dev_dependencies:
   build_runner: ^1.0.0
   build_verify: ^2.0.0
-  collection: ^1.14.0
-  dart_style: ^1.2.0
+  dart_style: ^2.0.0
   logging: ^1.0.0
   pub_semver: ^2.0.0
-  source_gen_test: ^0.1.0
+  source_gen_test: ^1.0.0
   stack_trace: ^1.10.0
   test: ^1.16.0
   yaml: ^3.0.0
+
+dependency_overrides:
+  json_annotation:
+    path: ../json_annotation
diff --git a/json_serializable/test/config_test.dart b/json_serializable/test/config_test.dart
index 7356a47f6..065aead27 100644
--- a/json_serializable/test/config_test.dart
+++ b/json_serializable/test/config_test.dart
@@ -114,7 +114,7 @@ void main() {
         final lastLine = entry.key == 'field_rename'
             ? '`42` is not one of the supported values: none, kebab, snake, '
                 'pascal'
-            : "type 'int' is not a subtype of type 'bool' in type cast";
+            : "type 'int' is not a subtype of type 'bool?' in type cast";
 
         final matcher = isA<StateError>().having(
           (v) => v.message,
diff --git a/json_serializable/test/custom_configuration_test.dart b/json_serializable/test/custom_configuration_test.dart
index 02a04cf1b..e9d9ab19d 100644
--- a/json_serializable/test/custom_configuration_test.dart
+++ b/json_serializable/test/custom_configuration_test.dart
@@ -18,7 +18,7 @@ import 'package:test/test.dart';
 
 import 'shared_config.dart';
 
-LibraryReader _libraryReader;
+late LibraryReader _libraryReader;
 
 Future<void> main() async {
   initializeBuildLogTracking();
@@ -36,7 +36,7 @@ Future<void> main() async {
 
   group('configuration', () {
     Future<void> runWithConfigAndLogger(
-        JsonSerializable config, String className) async {
+        JsonSerializable? config, String className) async {
       await generateForElement(
           JsonSerializableGenerator(
               config: config, typeHelpers: const [_ConfigLogger()]),
@@ -214,7 +214,7 @@ class _ConfigLogger implements TypeHelper<TypeHelperContextWithConfig> {
   const _ConfigLogger();
 
   @override
-  Object deserialize(
+  Object? deserialize(
     DartType targetType,
     String expression,
     TypeHelperContextWithConfig context,
@@ -225,7 +225,7 @@ class _ConfigLogger implements TypeHelper<TypeHelperContextWithConfig> {
   }
 
   @override
-  Object serialize(DartType targetType, String expression,
+  Object? serialize(DartType targetType, String expression,
       TypeHelperContextWithConfig context) {
     configurations.add(context.config);
     return null;
diff --git a/json_serializable/test/default_value/default_value.dart b/json_serializable/test/default_value/default_value.dart
index 84f1b972e..27eda4ea6 100644
--- a/json_serializable/test/default_value/default_value.dart
+++ b/json_serializable/test/default_value/default_value.dart
@@ -2,8 +2,6 @@
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
-// @dart=2.12
-
 // ignore_for_file: annotate_overrides
 
 import 'package:json_annotation/json_annotation.dart';
diff --git a/json_serializable/test/default_value/default_value.g.dart b/json_serializable/test/default_value/default_value.g.dart
index 0c29c64b8..fbf121f1d 100644
--- a/json_serializable/test/default_value/default_value.g.dart
+++ b/json_serializable/test/default_value/default_value.g.dart
@@ -1,5 +1,4 @@
 // GENERATED CODE - DO NOT MODIFY BY HAND
-// @dart=2.12
 
 part of 'default_value.dart';
 
diff --git a/json_serializable/test/default_value/default_value.g_any_map__checked.dart b/json_serializable/test/default_value/default_value.g_any_map__checked.dart
index 615505145..9c345aef1 100644
--- a/json_serializable/test/default_value/default_value.g_any_map__checked.dart
+++ b/json_serializable/test/default_value/default_value.g_any_map__checked.dart
@@ -2,8 +2,6 @@
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
-// @dart=2.12
-
 // ignore_for_file: annotate_overrides
 
 import 'package:json_annotation/json_annotation.dart';
diff --git a/json_serializable/test/default_value/default_value.g_any_map__checked.g.dart b/json_serializable/test/default_value/default_value.g_any_map__checked.g.dart
index 5aa38d430..678bab817 100644
--- a/json_serializable/test/default_value/default_value.g_any_map__checked.g.dart
+++ b/json_serializable/test/default_value/default_value.g_any_map__checked.g.dart
@@ -1,5 +1,4 @@
 // GENERATED CODE - DO NOT MODIFY BY HAND
-// @dart=2.12
 
 part of 'default_value.g_any_map__checked.dart';
 
diff --git a/json_serializable/test/default_value/default_value_interface.dart b/json_serializable/test/default_value/default_value_interface.dart
index 65076e6eb..06bc33408 100644
--- a/json_serializable/test/default_value/default_value_interface.dart
+++ b/json_serializable/test/default_value/default_value_interface.dart
@@ -2,8 +2,6 @@
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
-// @dart=2.12
-
 abstract class DefaultValue {
   bool fieldBool;
   String fieldString;
diff --git a/json_serializable/test/default_value/default_value_test.dart b/json_serializable/test/default_value/default_value_test.dart
index 8a0a83ff1..00ef236ad 100644
--- a/json_serializable/test/default_value/default_value_test.dart
+++ b/json_serializable/test/default_value/default_value_test.dart
@@ -2,8 +2,6 @@
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
-// @dart=2.12
-
 import 'package:test/test.dart';
 
 import '../test_utils.dart';
diff --git a/json_serializable/test/generic_files/generic_argument_factories.dart b/json_serializable/test/generic_files/generic_argument_factories.dart
index c45ebf081..0ebb09d69 100644
--- a/json_serializable/test/generic_files/generic_argument_factories.dart
+++ b/json_serializable/test/generic_files/generic_argument_factories.dart
@@ -2,8 +2,6 @@
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
-// @dart=2.12
-
 import 'package:json_annotation/json_annotation.dart';
 
 part 'generic_argument_factories.g.dart';
diff --git a/json_serializable/test/generic_files/generic_argument_factories.g.dart b/json_serializable/test/generic_files/generic_argument_factories.g.dart
index 49ba5d532..da41c86ff 100644
--- a/json_serializable/test/generic_files/generic_argument_factories.g.dart
+++ b/json_serializable/test/generic_files/generic_argument_factories.g.dart
@@ -1,5 +1,4 @@
 // GENERATED CODE - DO NOT MODIFY BY HAND
-// @dart=2.12
 
 part of 'generic_argument_factories.dart';
 
diff --git a/json_serializable/test/generic_files/generic_class.dart b/json_serializable/test/generic_files/generic_class.dart
index 4b55add02..bf47a098d 100644
--- a/json_serializable/test/generic_files/generic_class.dart
+++ b/json_serializable/test/generic_files/generic_class.dart
@@ -2,8 +2,6 @@
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
-// @dart=2.12
-
 import 'package:json_annotation/json_annotation.dart';
 
 part 'generic_class.g.dart';
diff --git a/json_serializable/test/generic_files/generic_class.g.dart b/json_serializable/test/generic_files/generic_class.g.dart
index 2ef18b32d..d1fb01b0f 100644
--- a/json_serializable/test/generic_files/generic_class.g.dart
+++ b/json_serializable/test/generic_files/generic_class.g.dart
@@ -1,5 +1,4 @@
 // GENERATED CODE - DO NOT MODIFY BY HAND
-// @dart=2.12
 
 part of 'generic_class.dart';
 
diff --git a/json_serializable/test/generic_files/generic_test.dart b/json_serializable/test/generic_files/generic_test.dart
index b9e62c090..98b7e770b 100644
--- a/json_serializable/test/generic_files/generic_test.dart
+++ b/json_serializable/test/generic_files/generic_test.dart
@@ -2,8 +2,6 @@
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
-// @dart=2.12
-
 import 'dart:convert';
 
 import 'package:test/test.dart';
diff --git a/json_serializable/test/integration/integration_test.dart b/json_serializable/test/integration/integration_test.dart
index 58379af97..c86f4ffbf 100644
--- a/json_serializable/test/integration/integration_test.dart
+++ b/json_serializable/test/integration/integration_test.dart
@@ -2,8 +2,6 @@
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
-// @dart=2.12
-
 import 'package:test/test.dart';
 
 import '../test_utils.dart';
diff --git a/json_serializable/test/integration/json_test_common.dart b/json_serializable/test/integration/json_test_common.dart
index 931d0ca14..6cd110a4d 100644
--- a/json_serializable/test/integration/json_test_common.dart
+++ b/json_serializable/test/integration/json_test_common.dart
@@ -2,8 +2,6 @@
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
-// @dart=2.12
-
 import 'dart:collection';
 
 import 'package:collection/collection.dart';
diff --git a/json_serializable/test/integration/json_test_example.dart b/json_serializable/test/integration/json_test_example.dart
index 4a1fcec0f..6a30846fe 100644
--- a/json_serializable/test/integration/json_test_example.dart
+++ b/json_serializable/test/integration/json_test_example.dart
@@ -2,8 +2,6 @@
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
-// @dart=2.12
-
 // ignore_for_file: hash_and_equals
 import 'dart:collection';
 
diff --git a/json_serializable/test/integration/json_test_example.g.dart b/json_serializable/test/integration/json_test_example.g.dart
index a9f5d28e2..da899ac94 100644
--- a/json_serializable/test/integration/json_test_example.g.dart
+++ b/json_serializable/test/integration/json_test_example.g.dart
@@ -1,5 +1,4 @@
 // GENERATED CODE - DO NOT MODIFY BY HAND
-// @dart=2.12
 
 part of 'json_test_example.dart';
 
diff --git a/json_serializable/test/integration/json_test_example.g_any_map.dart b/json_serializable/test/integration/json_test_example.g_any_map.dart
index 1796bd37f..c61b891bc 100644
--- a/json_serializable/test/integration/json_test_example.g_any_map.dart
+++ b/json_serializable/test/integration/json_test_example.g_any_map.dart
@@ -2,8 +2,6 @@
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
-// @dart=2.12
-
 // ignore_for_file: hash_and_equals
 import 'dart:collection';
 
diff --git a/json_serializable/test/integration/json_test_example.g_any_map.g.dart b/json_serializable/test/integration/json_test_example.g_any_map.g.dart
index ef4a6566d..bfa0c1378 100644
--- a/json_serializable/test/integration/json_test_example.g_any_map.g.dart
+++ b/json_serializable/test/integration/json_test_example.g_any_map.g.dart
@@ -1,5 +1,4 @@
 // GENERATED CODE - DO NOT MODIFY BY HAND
-// @dart=2.12
 
 part of 'json_test_example.g_any_map.dart';
 
diff --git a/json_serializable/test/kitchen_sink/json_converters.dart b/json_serializable/test/kitchen_sink/json_converters.dart
index c5074416f..aefb246d2 100644
--- a/json_serializable/test/kitchen_sink/json_converters.dart
+++ b/json_serializable/test/kitchen_sink/json_converters.dart
@@ -2,8 +2,6 @@
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
-// @dart=2.12
-
 import 'package:json_annotation/json_annotation.dart';
 
 class GenericConverter<T> implements JsonConverter<T, Map<String, dynamic>> {
diff --git a/json_serializable/test/kitchen_sink/kitchen_sink.dart b/json_serializable/test/kitchen_sink/kitchen_sink.dart
index 113fe5e84..d846eee3d 100644
--- a/json_serializable/test/kitchen_sink/kitchen_sink.dart
+++ b/json_serializable/test/kitchen_sink/kitchen_sink.dart
@@ -2,8 +2,6 @@
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
-// @dart=2.12
-
 // ignore_for_file: annotate_overrides, hash_and_equals
 import 'package:json_annotation/json_annotation.dart';
 
diff --git a/json_serializable/test/kitchen_sink/kitchen_sink.factories.dart b/json_serializable/test/kitchen_sink/kitchen_sink.factories.dart
index 3c26454da..863afa9bb 100644
--- a/json_serializable/test/kitchen_sink/kitchen_sink.factories.dart
+++ b/json_serializable/test/kitchen_sink/kitchen_sink.factories.dart
@@ -1,5 +1,4 @@
 // GENERATED CODE - DO NOT MODIFY BY HAND
-// @dart=2.12
 
 import 'kitchen_sink.dart' as normal;
 import 'kitchen_sink.g_any_map.dart' as any_map;
diff --git a/json_serializable/test/kitchen_sink/kitchen_sink.g.dart b/json_serializable/test/kitchen_sink/kitchen_sink.g.dart
index 027129a3a..03a894f51 100644
--- a/json_serializable/test/kitchen_sink/kitchen_sink.g.dart
+++ b/json_serializable/test/kitchen_sink/kitchen_sink.g.dart
@@ -1,5 +1,4 @@
 // GENERATED CODE - DO NOT MODIFY BY HAND
-// @dart=2.12
 
 part of 'kitchen_sink.dart';
 
diff --git a/json_serializable/test/kitchen_sink/kitchen_sink.g_any_map.dart b/json_serializable/test/kitchen_sink/kitchen_sink.g_any_map.dart
index 5d0ce3fbe..5db543802 100644
--- a/json_serializable/test/kitchen_sink/kitchen_sink.g_any_map.dart
+++ b/json_serializable/test/kitchen_sink/kitchen_sink.g_any_map.dart
@@ -2,8 +2,6 @@
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
-// @dart=2.12
-
 // ignore_for_file: annotate_overrides, hash_and_equals
 import 'package:json_annotation/json_annotation.dart';
 
diff --git a/json_serializable/test/kitchen_sink/kitchen_sink.g_any_map.g.dart b/json_serializable/test/kitchen_sink/kitchen_sink.g_any_map.g.dart
index 0c88a1cbd..01b13ef4b 100644
--- a/json_serializable/test/kitchen_sink/kitchen_sink.g_any_map.g.dart
+++ b/json_serializable/test/kitchen_sink/kitchen_sink.g_any_map.g.dart
@@ -1,5 +1,4 @@
 // GENERATED CODE - DO NOT MODIFY BY HAND
-// @dart=2.12
 
 part of 'kitchen_sink.g_any_map.dart';
 
diff --git a/json_serializable/test/kitchen_sink/kitchen_sink.g_any_map__checked.dart b/json_serializable/test/kitchen_sink/kitchen_sink.g_any_map__checked.dart
index 0c751950d..4782d07a7 100644
--- a/json_serializable/test/kitchen_sink/kitchen_sink.g_any_map__checked.dart
+++ b/json_serializable/test/kitchen_sink/kitchen_sink.g_any_map__checked.dart
@@ -2,8 +2,6 @@
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
-// @dart=2.12
-
 // ignore_for_file: annotate_overrides, hash_and_equals
 import 'package:json_annotation/json_annotation.dart';
 
diff --git a/json_serializable/test/kitchen_sink/kitchen_sink.g_any_map__checked.g.dart b/json_serializable/test/kitchen_sink/kitchen_sink.g_any_map__checked.g.dart
index 3ff15a43b..42562e25a 100644
--- a/json_serializable/test/kitchen_sink/kitchen_sink.g_any_map__checked.g.dart
+++ b/json_serializable/test/kitchen_sink/kitchen_sink.g_any_map__checked.g.dart
@@ -1,5 +1,4 @@
 // GENERATED CODE - DO NOT MODIFY BY HAND
-// @dart=2.12
 
 part of 'kitchen_sink.g_any_map__checked.dart';
 
diff --git a/json_serializable/test/kitchen_sink/kitchen_sink.g_exclude_null.dart b/json_serializable/test/kitchen_sink/kitchen_sink.g_exclude_null.dart
index 70c60859b..7456d12cd 100644
--- a/json_serializable/test/kitchen_sink/kitchen_sink.g_exclude_null.dart
+++ b/json_serializable/test/kitchen_sink/kitchen_sink.g_exclude_null.dart
@@ -2,8 +2,6 @@
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
-// @dart=2.12
-
 // ignore_for_file: annotate_overrides, hash_and_equals
 import 'package:json_annotation/json_annotation.dart';
 
diff --git a/json_serializable/test/kitchen_sink/kitchen_sink.g_exclude_null.g.dart b/json_serializable/test/kitchen_sink/kitchen_sink.g_exclude_null.g.dart
index fdc362702..095cd3892 100644
--- a/json_serializable/test/kitchen_sink/kitchen_sink.g_exclude_null.g.dart
+++ b/json_serializable/test/kitchen_sink/kitchen_sink.g_exclude_null.g.dart
@@ -1,5 +1,4 @@
 // GENERATED CODE - DO NOT MODIFY BY HAND
-// @dart=2.12
 
 part of 'kitchen_sink.g_exclude_null.dart';
 
diff --git a/json_serializable/test/kitchen_sink/kitchen_sink.g_explicit_to_json.dart b/json_serializable/test/kitchen_sink/kitchen_sink.g_explicit_to_json.dart
index 6bc665612..8f2875f59 100644
--- a/json_serializable/test/kitchen_sink/kitchen_sink.g_explicit_to_json.dart
+++ b/json_serializable/test/kitchen_sink/kitchen_sink.g_explicit_to_json.dart
@@ -2,8 +2,6 @@
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
-// @dart=2.12
-
 // ignore_for_file: annotate_overrides, hash_and_equals
 import 'package:json_annotation/json_annotation.dart';
 
diff --git a/json_serializable/test/kitchen_sink/kitchen_sink.g_explicit_to_json.g.dart b/json_serializable/test/kitchen_sink/kitchen_sink.g_explicit_to_json.g.dart
index 9dda090c9..24e14ac64 100644
--- a/json_serializable/test/kitchen_sink/kitchen_sink.g_explicit_to_json.g.dart
+++ b/json_serializable/test/kitchen_sink/kitchen_sink.g_explicit_to_json.g.dart
@@ -1,5 +1,4 @@
 // GENERATED CODE - DO NOT MODIFY BY HAND
-// @dart=2.12
 
 part of 'kitchen_sink.g_explicit_to_json.dart';
 
diff --git a/json_serializable/test/kitchen_sink/kitchen_sink_interface.dart b/json_serializable/test/kitchen_sink/kitchen_sink_interface.dart
index 2473ca1df..357085e5c 100644
--- a/json_serializable/test/kitchen_sink/kitchen_sink_interface.dart
+++ b/json_serializable/test/kitchen_sink/kitchen_sink_interface.dart
@@ -2,8 +2,6 @@
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
-// @dart=2.12
-
 import 'package:collection/collection.dart';
 
 import 'simple_object.dart';
diff --git a/json_serializable/test/kitchen_sink/kitchen_sink_test.dart b/json_serializable/test/kitchen_sink/kitchen_sink_test.dart
index 3950b66fa..b8566a9e5 100644
--- a/json_serializable/test/kitchen_sink/kitchen_sink_test.dart
+++ b/json_serializable/test/kitchen_sink/kitchen_sink_test.dart
@@ -2,8 +2,6 @@
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
-// @dart=2.12
-
 import 'package:json_annotation/json_annotation.dart';
 import 'package:test/test.dart';
 
diff --git a/json_serializable/test/kitchen_sink/kitchen_sink_test_shared.dart b/json_serializable/test/kitchen_sink/kitchen_sink_test_shared.dart
index 4b1b60bc0..f69535d6a 100644
--- a/json_serializable/test/kitchen_sink/kitchen_sink_test_shared.dart
+++ b/json_serializable/test/kitchen_sink/kitchen_sink_test_shared.dart
@@ -1,5 +1,3 @@
-// @dart=2.12
-
 import 'package:json_annotation/json_annotation.dart';
 import 'package:test/test.dart';
 
diff --git a/json_serializable/test/kitchen_sink/kitchen_sink_yaml_test.dart b/json_serializable/test/kitchen_sink/kitchen_sink_yaml_test.dart
index 55846778d..f9a09b521 100644
--- a/json_serializable/test/kitchen_sink/kitchen_sink_yaml_test.dart
+++ b/json_serializable/test/kitchen_sink/kitchen_sink_yaml_test.dart
@@ -1,5 +1,3 @@
-// @dart=2.12
-
 import 'package:json_annotation/json_annotation.dart';
 import 'package:test/test.dart';
 import 'package:yaml/yaml.dart';
diff --git a/json_serializable/test/kitchen_sink/simple_object.dart b/json_serializable/test/kitchen_sink/simple_object.dart
index b5816fc29..c02cefc31 100644
--- a/json_serializable/test/kitchen_sink/simple_object.dart
+++ b/json_serializable/test/kitchen_sink/simple_object.dart
@@ -2,8 +2,6 @@
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
-// @dart=2.12
-
 import 'package:json_annotation/json_annotation.dart';
 
 part 'simple_object.g.dart';
diff --git a/json_serializable/test/kitchen_sink/simple_object.g.dart b/json_serializable/test/kitchen_sink/simple_object.g.dart
index ad76883f5..42d584b69 100644
--- a/json_serializable/test/kitchen_sink/simple_object.g.dart
+++ b/json_serializable/test/kitchen_sink/simple_object.g.dart
@@ -1,5 +1,4 @@
 // GENERATED CODE - DO NOT MODIFY BY HAND
-// @dart=2.12
 
 part of 'simple_object.dart';
 
diff --git a/json_serializable/test/kitchen_sink/strict_keys_object.dart b/json_serializable/test/kitchen_sink/strict_keys_object.dart
index bc54f9ba8..757f3eb5c 100644
--- a/json_serializable/test/kitchen_sink/strict_keys_object.dart
+++ b/json_serializable/test/kitchen_sink/strict_keys_object.dart
@@ -2,8 +2,6 @@
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
-// @dart=2.12
-
 import 'package:json_annotation/json_annotation.dart';
 
 part 'strict_keys_object.g.dart';
diff --git a/json_serializable/test/kitchen_sink/strict_keys_object.g.dart b/json_serializable/test/kitchen_sink/strict_keys_object.g.dart
index 50dd9e5d6..dfdda8eaa 100644
--- a/json_serializable/test/kitchen_sink/strict_keys_object.g.dart
+++ b/json_serializable/test/kitchen_sink/strict_keys_object.g.dart
@@ -1,5 +1,4 @@
 // GENERATED CODE - DO NOT MODIFY BY HAND
-// @dart=2.12
 
 part of 'strict_keys_object.dart';
 
diff --git a/json_serializable/test/readme_test.dart b/json_serializable/test/readme_test.dart
index c11c50a98..24c1be8b9 100644
--- a/json_serializable/test/readme_test.dart
+++ b/json_serializable/test/readme_test.dart
@@ -9,7 +9,7 @@ import 'package:path/path.dart' as p;
 import 'package:test/test.dart';
 
 void main() {
-  String readmeContent;
+  late String readmeContent;
 
   setUpAll(() {
     readmeContent = File('README.md').readAsStringSync();
diff --git a/json_serializable/test/src/_json_serializable_test_input.dart b/json_serializable/test/src/_json_serializable_test_input.dart
index 7ba5c0f3f..81e6b03b7 100644
--- a/json_serializable/test/src/_json_serializable_test_input.dart
+++ b/json_serializable/test/src/_json_serializable_test_input.dart
@@ -2,8 +2,6 @@
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
-// @dart=2.12
-
 import 'dart:collection';
 
 import 'package:json_annotation/json_annotation.dart';
diff --git a/json_serializable/test/src/checked_test_input.dart b/json_serializable/test/src/checked_test_input.dart
index 7d3d17238..9a87630c5 100644
--- a/json_serializable/test/src/checked_test_input.dart
+++ b/json_serializable/test/src/checked_test_input.dart
@@ -1,5 +1,3 @@
-// @dart=2.12
-
 part of '_json_serializable_test_input.dart';
 
 @ShouldGenerate(r'''
diff --git a/json_serializable/test/src/constants_copy.dart b/json_serializable/test/src/constants_copy.dart
index 27b33fb51..a4b0f1086 100644
--- a/json_serializable/test/src/constants_copy.dart
+++ b/json_serializable/test/src/constants_copy.dart
@@ -2,8 +2,6 @@
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
-// @dart=2.12
-
 part of '_json_serializable_test_input.dart';
 
 // TODO: remove this and return link to lib/src/constants.dart once this
diff --git a/json_serializable/test/src/core_subclass_type_input.dart b/json_serializable/test/src/core_subclass_type_input.dart
index 8f58465cc..02b889a2c 100644
--- a/json_serializable/test/src/core_subclass_type_input.dart
+++ b/json_serializable/test/src/core_subclass_type_input.dart
@@ -1,5 +1,3 @@
-// @dart=2.12
-
 part of '_json_serializable_test_input.dart';
 
 @ShouldThrow(
diff --git a/json_serializable/test/src/default_value_input.dart b/json_serializable/test/src/default_value_input.dart
index aca8e90dd..f15739b2f 100644
--- a/json_serializable/test/src/default_value_input.dart
+++ b/json_serializable/test/src/default_value_input.dart
@@ -2,8 +2,6 @@
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
-// @dart=2.12
-
 part of '_json_serializable_test_input.dart';
 
 @ShouldThrow(
diff --git a/json_serializable/test/src/field_namer_input.dart b/json_serializable/test/src/field_namer_input.dart
index 1a0cb07ca..5f8bc6011 100644
--- a/json_serializable/test/src/field_namer_input.dart
+++ b/json_serializable/test/src/field_namer_input.dart
@@ -1,5 +1,3 @@
-// @dart=2.12
-
 part of '_json_serializable_test_input.dart';
 
 @ShouldGenerate(r'''
diff --git a/json_serializable/test/src/generic_test_input.dart b/json_serializable/test/src/generic_test_input.dart
index e60c395e9..1a3b98466 100644
--- a/json_serializable/test/src/generic_test_input.dart
+++ b/json_serializable/test/src/generic_test_input.dart
@@ -2,8 +2,6 @@
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
-// @dart=2.12
-
 part of '_json_serializable_test_input.dart';
 
 @ShouldThrow(
diff --git a/json_serializable/test/src/inheritance_test_input.dart b/json_serializable/test/src/inheritance_test_input.dart
index 5d0294408..c853be117 100644
--- a/json_serializable/test/src/inheritance_test_input.dart
+++ b/json_serializable/test/src/inheritance_test_input.dart
@@ -1,5 +1,3 @@
-// @dart=2.12
-
 part of '_json_serializable_test_input.dart';
 
 @ShouldGenerate(r'''
diff --git a/json_serializable/test/src/json_converter_test_input.dart b/json_serializable/test/src/json_converter_test_input.dart
index df58ecd00..9086bccff 100644
--- a/json_serializable/test/src/json_converter_test_input.dart
+++ b/json_serializable/test/src/json_converter_test_input.dart
@@ -2,8 +2,6 @@
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
-// @dart=2.12
-
 part of '_json_serializable_test_input.dart';
 
 @ShouldGenerate(r'''
diff --git a/json_serializable/test/src/map_key_variety_test_input.dart b/json_serializable/test/src/map_key_variety_test_input.dart
index 4f022d58a..68504cd76 100644
--- a/json_serializable/test/src/map_key_variety_test_input.dart
+++ b/json_serializable/test/src/map_key_variety_test_input.dart
@@ -1,5 +1,3 @@
-// @dart=2.12
-
 part of '_json_serializable_test_input.dart';
 
 @ShouldGenerate(r'''
diff --git a/json_serializable/test/src/setter_test_input.dart b/json_serializable/test/src/setter_test_input.dart
index 9fe2d890e..b951dfeb5 100644
--- a/json_serializable/test/src/setter_test_input.dart
+++ b/json_serializable/test/src/setter_test_input.dart
@@ -1,5 +1,3 @@
-// @dart=2.12
-
 part of '_json_serializable_test_input.dart';
 
 @ShouldGenerate(
diff --git a/json_serializable/test/src/to_from_json_test_input.dart b/json_serializable/test/src/to_from_json_test_input.dart
index 321146901..40c997e3a 100644
--- a/json_serializable/test/src/to_from_json_test_input.dart
+++ b/json_serializable/test/src/to_from_json_test_input.dart
@@ -2,8 +2,6 @@
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
-// @dart=2.12
-
 part of '_json_serializable_test_input.dart';
 
 int _toInt(bool input) => 42;
diff --git a/json_serializable/test/src/unknown_enum_value_test_input.dart b/json_serializable/test/src/unknown_enum_value_test_input.dart
index 0f6907087..ea238231d 100644
--- a/json_serializable/test/src/unknown_enum_value_test_input.dart
+++ b/json_serializable/test/src/unknown_enum_value_test_input.dart
@@ -1,5 +1,3 @@
-// @dart=2.12
-
 part of '_json_serializable_test_input.dart';
 
 @ShouldGenerate(
diff --git a/json_serializable/test/supported_types/enum_type.dart b/json_serializable/test/supported_types/enum_type.dart
index a0f9c2191..360a6bf8e 100644
--- a/json_serializable/test/supported_types/enum_type.dart
+++ b/json_serializable/test/supported_types/enum_type.dart
@@ -2,6 +2,4 @@
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
-// @dart=2.12
-
 enum EnumType { alpha, beta, gamma, delta }
diff --git a/json_serializable/test/supported_types/input.dart b/json_serializable/test/supported_types/input.dart
index 2319b01c0..8a43e9fcc 100644
--- a/json_serializable/test/supported_types/input.dart
+++ b/json_serializable/test/supported_types/input.dart
@@ -2,8 +2,6 @@
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
-// @dart=2.12
-
 import 'package:json_annotation/json_annotation.dart';
 
 part 'input.g.dart';
diff --git a/json_serializable/test/supported_types/input.g.dart b/json_serializable/test/supported_types/input.g.dart
index 65367bf60..3effa4548 100644
--- a/json_serializable/test/supported_types/input.g.dart
+++ b/json_serializable/test/supported_types/input.g.dart
@@ -1,5 +1,4 @@
 // GENERATED CODE - DO NOT MODIFY BY HAND
-// @dart=2.12
 
 part of 'input.dart';
 
diff --git a/json_serializable/test/supported_types/input.type_bigint.dart b/json_serializable/test/supported_types/input.type_bigint.dart
index 25005752e..07ab37ba4 100644
--- a/json_serializable/test/supported_types/input.type_bigint.dart
+++ b/json_serializable/test/supported_types/input.type_bigint.dart
@@ -2,8 +2,6 @@
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
-// @dart=2.12
-
 import 'package:json_annotation/json_annotation.dart';
 
 part 'input.type_bigint.g.dart';
diff --git a/json_serializable/test/supported_types/input.type_bigint.g.dart b/json_serializable/test/supported_types/input.type_bigint.g.dart
index c03d1ed5f..c22bc2d85 100644
--- a/json_serializable/test/supported_types/input.type_bigint.g.dart
+++ b/json_serializable/test/supported_types/input.type_bigint.g.dart
@@ -1,5 +1,4 @@
 // GENERATED CODE - DO NOT MODIFY BY HAND
-// @dart=2.12
 
 part of 'input.type_bigint.dart';
 
diff --git a/json_serializable/test/supported_types/input.type_bool.dart b/json_serializable/test/supported_types/input.type_bool.dart
index 9d898ba07..e3fd6078c 100644
--- a/json_serializable/test/supported_types/input.type_bool.dart
+++ b/json_serializable/test/supported_types/input.type_bool.dart
@@ -2,8 +2,6 @@
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
-// @dart=2.12
-
 import 'package:json_annotation/json_annotation.dart';
 
 part 'input.type_bool.g.dart';
diff --git a/json_serializable/test/supported_types/input.type_bool.g.dart b/json_serializable/test/supported_types/input.type_bool.g.dart
index 04a942e20..3a793eccc 100644
--- a/json_serializable/test/supported_types/input.type_bool.g.dart
+++ b/json_serializable/test/supported_types/input.type_bool.g.dart
@@ -1,5 +1,4 @@
 // GENERATED CODE - DO NOT MODIFY BY HAND
-// @dart=2.12
 
 part of 'input.type_bool.dart';
 
diff --git a/json_serializable/test/supported_types/input.type_datetime.dart b/json_serializable/test/supported_types/input.type_datetime.dart
index cc992713e..880c7ce89 100644
--- a/json_serializable/test/supported_types/input.type_datetime.dart
+++ b/json_serializable/test/supported_types/input.type_datetime.dart
@@ -2,8 +2,6 @@
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
-// @dart=2.12
-
 import 'package:json_annotation/json_annotation.dart';
 
 part 'input.type_datetime.g.dart';
diff --git a/json_serializable/test/supported_types/input.type_datetime.g.dart b/json_serializable/test/supported_types/input.type_datetime.g.dart
index b98a2c17d..2f3d8bd9a 100644
--- a/json_serializable/test/supported_types/input.type_datetime.g.dart
+++ b/json_serializable/test/supported_types/input.type_datetime.g.dart
@@ -1,5 +1,4 @@
 // GENERATED CODE - DO NOT MODIFY BY HAND
-// @dart=2.12
 
 part of 'input.type_datetime.dart';
 
diff --git a/json_serializable/test/supported_types/input.type_double.dart b/json_serializable/test/supported_types/input.type_double.dart
index e2074ddcd..c51a0e35b 100644
--- a/json_serializable/test/supported_types/input.type_double.dart
+++ b/json_serializable/test/supported_types/input.type_double.dart
@@ -2,8 +2,6 @@
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
-// @dart=2.12
-
 import 'package:json_annotation/json_annotation.dart';
 
 part 'input.type_double.g.dart';
diff --git a/json_serializable/test/supported_types/input.type_double.g.dart b/json_serializable/test/supported_types/input.type_double.g.dart
index 00d0e424b..1ff5d3cb2 100644
--- a/json_serializable/test/supported_types/input.type_double.g.dart
+++ b/json_serializable/test/supported_types/input.type_double.g.dart
@@ -1,5 +1,4 @@
 // GENERATED CODE - DO NOT MODIFY BY HAND
-// @dart=2.12
 
 part of 'input.type_double.dart';
 
diff --git a/json_serializable/test/supported_types/input.type_duration.dart b/json_serializable/test/supported_types/input.type_duration.dart
index aa4c716e4..7ecdef825 100644
--- a/json_serializable/test/supported_types/input.type_duration.dart
+++ b/json_serializable/test/supported_types/input.type_duration.dart
@@ -2,8 +2,6 @@
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
-// @dart=2.12
-
 import 'package:json_annotation/json_annotation.dart';
 
 part 'input.type_duration.g.dart';
diff --git a/json_serializable/test/supported_types/input.type_duration.g.dart b/json_serializable/test/supported_types/input.type_duration.g.dart
index 5d7f2f45f..d2a77f769 100644
--- a/json_serializable/test/supported_types/input.type_duration.g.dart
+++ b/json_serializable/test/supported_types/input.type_duration.g.dart
@@ -1,5 +1,4 @@
 // GENERATED CODE - DO NOT MODIFY BY HAND
-// @dart=2.12
 
 part of 'input.type_duration.dart';
 
diff --git a/json_serializable/test/supported_types/input.type_enumtype.dart b/json_serializable/test/supported_types/input.type_enumtype.dart
index 14c28a906..9225db715 100644
--- a/json_serializable/test/supported_types/input.type_enumtype.dart
+++ b/json_serializable/test/supported_types/input.type_enumtype.dart
@@ -2,8 +2,6 @@
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
-// @dart=2.12
-
 import 'package:json_annotation/json_annotation.dart';
 import 'enum_type.dart';
 
diff --git a/json_serializable/test/supported_types/input.type_enumtype.g.dart b/json_serializable/test/supported_types/input.type_enumtype.g.dart
index 547664f35..7233add35 100644
--- a/json_serializable/test/supported_types/input.type_enumtype.g.dart
+++ b/json_serializable/test/supported_types/input.type_enumtype.g.dart
@@ -1,5 +1,4 @@
 // GENERATED CODE - DO NOT MODIFY BY HAND
-// @dart=2.12
 
 part of 'input.type_enumtype.dart';
 
diff --git a/json_serializable/test/supported_types/input.type_int.dart b/json_serializable/test/supported_types/input.type_int.dart
index 0a2614f22..6d80dc79c 100644
--- a/json_serializable/test/supported_types/input.type_int.dart
+++ b/json_serializable/test/supported_types/input.type_int.dart
@@ -2,8 +2,6 @@
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
-// @dart=2.12
-
 import 'package:json_annotation/json_annotation.dart';
 
 part 'input.type_int.g.dart';
diff --git a/json_serializable/test/supported_types/input.type_int.g.dart b/json_serializable/test/supported_types/input.type_int.g.dart
index 48295e2b7..c4695d106 100644
--- a/json_serializable/test/supported_types/input.type_int.g.dart
+++ b/json_serializable/test/supported_types/input.type_int.g.dart
@@ -1,5 +1,4 @@
 // GENERATED CODE - DO NOT MODIFY BY HAND
-// @dart=2.12
 
 part of 'input.type_int.dart';
 
diff --git a/json_serializable/test/supported_types/input.type_iterable.dart b/json_serializable/test/supported_types/input.type_iterable.dart
index a90c4d99c..1471dc935 100644
--- a/json_serializable/test/supported_types/input.type_iterable.dart
+++ b/json_serializable/test/supported_types/input.type_iterable.dart
@@ -2,8 +2,6 @@
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
-// @dart=2.12
-
 import 'package:json_annotation/json_annotation.dart';
 import 'enum_type.dart';
 
diff --git a/json_serializable/test/supported_types/input.type_iterable.g.dart b/json_serializable/test/supported_types/input.type_iterable.g.dart
index e0b5389ad..97766b6b8 100644
--- a/json_serializable/test/supported_types/input.type_iterable.g.dart
+++ b/json_serializable/test/supported_types/input.type_iterable.g.dart
@@ -1,5 +1,4 @@
 // GENERATED CODE - DO NOT MODIFY BY HAND
-// @dart=2.12
 
 part of 'input.type_iterable.dart';
 
diff --git a/json_serializable/test/supported_types/input.type_list.dart b/json_serializable/test/supported_types/input.type_list.dart
index 657d45296..41f35fe72 100644
--- a/json_serializable/test/supported_types/input.type_list.dart
+++ b/json_serializable/test/supported_types/input.type_list.dart
@@ -2,8 +2,6 @@
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
-// @dart=2.12
-
 import 'package:json_annotation/json_annotation.dart';
 import 'enum_type.dart';
 
diff --git a/json_serializable/test/supported_types/input.type_list.g.dart b/json_serializable/test/supported_types/input.type_list.g.dart
index 9c1b1a63c..bd7eeb756 100644
--- a/json_serializable/test/supported_types/input.type_list.g.dart
+++ b/json_serializable/test/supported_types/input.type_list.g.dart
@@ -1,5 +1,4 @@
 // GENERATED CODE - DO NOT MODIFY BY HAND
-// @dart=2.12
 
 part of 'input.type_list.dart';
 
diff --git a/json_serializable/test/supported_types/input.type_map.dart b/json_serializable/test/supported_types/input.type_map.dart
index 74243a4b1..bdf45e336 100644
--- a/json_serializable/test/supported_types/input.type_map.dart
+++ b/json_serializable/test/supported_types/input.type_map.dart
@@ -2,8 +2,6 @@
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
-// @dart=2.12
-
 import 'package:json_annotation/json_annotation.dart';
 import 'enum_type.dart';
 
diff --git a/json_serializable/test/supported_types/input.type_map.g.dart b/json_serializable/test/supported_types/input.type_map.g.dart
index dfa1c2f7b..55608e727 100644
--- a/json_serializable/test/supported_types/input.type_map.g.dart
+++ b/json_serializable/test/supported_types/input.type_map.g.dart
@@ -1,5 +1,4 @@
 // GENERATED CODE - DO NOT MODIFY BY HAND
-// @dart=2.12
 
 part of 'input.type_map.dart';
 
diff --git a/json_serializable/test/supported_types/input.type_num.dart b/json_serializable/test/supported_types/input.type_num.dart
index 5e9657066..57631ce62 100644
--- a/json_serializable/test/supported_types/input.type_num.dart
+++ b/json_serializable/test/supported_types/input.type_num.dart
@@ -2,8 +2,6 @@
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
-// @dart=2.12
-
 import 'package:json_annotation/json_annotation.dart';
 
 part 'input.type_num.g.dart';
diff --git a/json_serializable/test/supported_types/input.type_num.g.dart b/json_serializable/test/supported_types/input.type_num.g.dart
index d91fb9d68..0456261f1 100644
--- a/json_serializable/test/supported_types/input.type_num.g.dart
+++ b/json_serializable/test/supported_types/input.type_num.g.dart
@@ -1,5 +1,4 @@
 // GENERATED CODE - DO NOT MODIFY BY HAND
-// @dart=2.12
 
 part of 'input.type_num.dart';
 
diff --git a/json_serializable/test/supported_types/input.type_object.dart b/json_serializable/test/supported_types/input.type_object.dart
index fb0c2e860..193bd440a 100644
--- a/json_serializable/test/supported_types/input.type_object.dart
+++ b/json_serializable/test/supported_types/input.type_object.dart
@@ -2,8 +2,6 @@
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
-// @dart=2.12
-
 import 'package:json_annotation/json_annotation.dart';
 
 part 'input.type_object.g.dart';
diff --git a/json_serializable/test/supported_types/input.type_object.g.dart b/json_serializable/test/supported_types/input.type_object.g.dart
index 973229f41..691a85539 100644
--- a/json_serializable/test/supported_types/input.type_object.g.dart
+++ b/json_serializable/test/supported_types/input.type_object.g.dart
@@ -1,5 +1,4 @@
 // GENERATED CODE - DO NOT MODIFY BY HAND
-// @dart=2.12
 
 part of 'input.type_object.dart';
 
diff --git a/json_serializable/test/supported_types/input.type_set.dart b/json_serializable/test/supported_types/input.type_set.dart
index ea7eacd67..8e74fb7c2 100644
--- a/json_serializable/test/supported_types/input.type_set.dart
+++ b/json_serializable/test/supported_types/input.type_set.dart
@@ -2,8 +2,6 @@
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
-// @dart=2.12
-
 import 'package:json_annotation/json_annotation.dart';
 import 'enum_type.dart';
 
diff --git a/json_serializable/test/supported_types/input.type_set.g.dart b/json_serializable/test/supported_types/input.type_set.g.dart
index 43bf51e4a..32e27b0e6 100644
--- a/json_serializable/test/supported_types/input.type_set.g.dart
+++ b/json_serializable/test/supported_types/input.type_set.g.dart
@@ -1,5 +1,4 @@
 // GENERATED CODE - DO NOT MODIFY BY HAND
-// @dart=2.12
 
 part of 'input.type_set.dart';
 
diff --git a/json_serializable/test/supported_types/input.type_string.dart b/json_serializable/test/supported_types/input.type_string.dart
index 1131e9556..d199b13c0 100644
--- a/json_serializable/test/supported_types/input.type_string.dart
+++ b/json_serializable/test/supported_types/input.type_string.dart
@@ -2,8 +2,6 @@
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
-// @dart=2.12
-
 import 'package:json_annotation/json_annotation.dart';
 
 part 'input.type_string.g.dart';
diff --git a/json_serializable/test/supported_types/input.type_string.g.dart b/json_serializable/test/supported_types/input.type_string.g.dart
index 55ff7b924..54ef11b44 100644
--- a/json_serializable/test/supported_types/input.type_string.g.dart
+++ b/json_serializable/test/supported_types/input.type_string.g.dart
@@ -1,5 +1,4 @@
 // GENERATED CODE - DO NOT MODIFY BY HAND
-// @dart=2.12
 
 part of 'input.type_string.dart';
 
diff --git a/json_serializable/test/supported_types/input.type_uri.dart b/json_serializable/test/supported_types/input.type_uri.dart
index 15b2c39f0..fe625bbff 100644
--- a/json_serializable/test/supported_types/input.type_uri.dart
+++ b/json_serializable/test/supported_types/input.type_uri.dart
@@ -2,8 +2,6 @@
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
-// @dart=2.12
-
 import 'package:json_annotation/json_annotation.dart';
 
 part 'input.type_uri.g.dart';
diff --git a/json_serializable/test/supported_types/input.type_uri.g.dart b/json_serializable/test/supported_types/input.type_uri.g.dart
index d0d8e1daf..8392c64b3 100644
--- a/json_serializable/test/supported_types/input.type_uri.g.dart
+++ b/json_serializable/test/supported_types/input.type_uri.g.dart
@@ -1,5 +1,4 @@
 // GENERATED CODE - DO NOT MODIFY BY HAND
-// @dart=2.12
 
 part of 'input.type_uri.dart';
 
diff --git a/json_serializable/test/supported_types/type_test.bigint_test.dart b/json_serializable/test/supported_types/type_test.bigint_test.dart
index 969cadb27..c012ea507 100644
--- a/json_serializable/test/supported_types/type_test.bigint_test.dart
+++ b/json_serializable/test/supported_types/type_test.bigint_test.dart
@@ -2,8 +2,6 @@
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
-// @dart=2.12
-
 // ignore_for_file: prefer_const_declarations
 
 import 'dart:convert';
diff --git a/json_serializable/test/supported_types/type_test.bool_test.dart b/json_serializable/test/supported_types/type_test.bool_test.dart
index 31e4f7971..14e24853b 100644
--- a/json_serializable/test/supported_types/type_test.bool_test.dart
+++ b/json_serializable/test/supported_types/type_test.bool_test.dart
@@ -2,8 +2,6 @@
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
-// @dart=2.12
-
 // ignore_for_file: prefer_const_declarations
 
 import 'dart:convert';
diff --git a/json_serializable/test/supported_types/type_test.dart b/json_serializable/test/supported_types/type_test.dart
index 4cafc262b..625b8e3c4 100644
--- a/json_serializable/test/supported_types/type_test.dart
+++ b/json_serializable/test/supported_types/type_test.dart
@@ -2,8 +2,6 @@
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
-// @dart=2.12
-
 // ignore_for_file: prefer_const_declarations
 
 import 'dart:convert';
diff --git a/json_serializable/test/supported_types/type_test.datetime_test.dart b/json_serializable/test/supported_types/type_test.datetime_test.dart
index 742529c2a..597bbf680 100644
--- a/json_serializable/test/supported_types/type_test.datetime_test.dart
+++ b/json_serializable/test/supported_types/type_test.datetime_test.dart
@@ -2,8 +2,6 @@
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
-// @dart=2.12
-
 // ignore_for_file: prefer_const_declarations
 
 import 'dart:convert';
diff --git a/json_serializable/test/supported_types/type_test.double_test.dart b/json_serializable/test/supported_types/type_test.double_test.dart
index c718f6c11..3e4c05acf 100644
--- a/json_serializable/test/supported_types/type_test.double_test.dart
+++ b/json_serializable/test/supported_types/type_test.double_test.dart
@@ -2,8 +2,6 @@
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
-// @dart=2.12
-
 // ignore_for_file: prefer_const_declarations
 
 import 'dart:convert';
diff --git a/json_serializable/test/supported_types/type_test.duration_test.dart b/json_serializable/test/supported_types/type_test.duration_test.dart
index ee91fd475..76816e9fc 100644
--- a/json_serializable/test/supported_types/type_test.duration_test.dart
+++ b/json_serializable/test/supported_types/type_test.duration_test.dart
@@ -2,8 +2,6 @@
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
-// @dart=2.12
-
 // ignore_for_file: prefer_const_declarations
 
 import 'dart:convert';
diff --git a/json_serializable/test/supported_types/type_test.enumtype_test.dart b/json_serializable/test/supported_types/type_test.enumtype_test.dart
index 5ead9e9f2..1652b359b 100644
--- a/json_serializable/test/supported_types/type_test.enumtype_test.dart
+++ b/json_serializable/test/supported_types/type_test.enumtype_test.dart
@@ -2,8 +2,6 @@
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
-// @dart=2.12
-
 // ignore_for_file: prefer_const_declarations
 
 import 'dart:convert';
diff --git a/json_serializable/test/supported_types/type_test.int_test.dart b/json_serializable/test/supported_types/type_test.int_test.dart
index 8ec0cd571..b4252fa5f 100644
--- a/json_serializable/test/supported_types/type_test.int_test.dart
+++ b/json_serializable/test/supported_types/type_test.int_test.dart
@@ -2,8 +2,6 @@
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
-// @dart=2.12
-
 // ignore_for_file: prefer_const_declarations
 
 import 'dart:convert';
diff --git a/json_serializable/test/supported_types/type_test.iterable_test.dart b/json_serializable/test/supported_types/type_test.iterable_test.dart
index cad1cbb99..57b9358b0 100644
--- a/json_serializable/test/supported_types/type_test.iterable_test.dart
+++ b/json_serializable/test/supported_types/type_test.iterable_test.dart
@@ -2,8 +2,6 @@
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
-// @dart=2.12
-
 // ignore_for_file: prefer_const_declarations
 
 import 'dart:convert';
diff --git a/json_serializable/test/supported_types/type_test.list_test.dart b/json_serializable/test/supported_types/type_test.list_test.dart
index ebfe17126..342a89079 100644
--- a/json_serializable/test/supported_types/type_test.list_test.dart
+++ b/json_serializable/test/supported_types/type_test.list_test.dart
@@ -2,8 +2,6 @@
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
-// @dart=2.12
-
 // ignore_for_file: prefer_const_declarations
 
 import 'dart:convert';
diff --git a/json_serializable/test/supported_types/type_test.map_test.dart b/json_serializable/test/supported_types/type_test.map_test.dart
index 2691408d1..c8cf4ccb9 100644
--- a/json_serializable/test/supported_types/type_test.map_test.dart
+++ b/json_serializable/test/supported_types/type_test.map_test.dart
@@ -2,8 +2,6 @@
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
-// @dart=2.12
-
 // ignore_for_file: prefer_const_declarations
 
 import 'dart:convert';
diff --git a/json_serializable/test/supported_types/type_test.num_test.dart b/json_serializable/test/supported_types/type_test.num_test.dart
index a72a873a3..52241e719 100644
--- a/json_serializable/test/supported_types/type_test.num_test.dart
+++ b/json_serializable/test/supported_types/type_test.num_test.dart
@@ -2,8 +2,6 @@
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
-// @dart=2.12
-
 // ignore_for_file: prefer_const_declarations
 
 import 'dart:convert';
diff --git a/json_serializable/test/supported_types/type_test.object_test.dart b/json_serializable/test/supported_types/type_test.object_test.dart
index ef38a0446..a4f9601f6 100644
--- a/json_serializable/test/supported_types/type_test.object_test.dart
+++ b/json_serializable/test/supported_types/type_test.object_test.dart
@@ -2,8 +2,6 @@
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
-// @dart=2.12
-
 // ignore_for_file: prefer_const_declarations
 
 import 'dart:convert';
diff --git a/json_serializable/test/supported_types/type_test.set_test.dart b/json_serializable/test/supported_types/type_test.set_test.dart
index 99fff3b6b..a3b1ed57b 100644
--- a/json_serializable/test/supported_types/type_test.set_test.dart
+++ b/json_serializable/test/supported_types/type_test.set_test.dart
@@ -2,8 +2,6 @@
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
-// @dart=2.12
-
 // ignore_for_file: prefer_const_declarations
 
 import 'dart:convert';
diff --git a/json_serializable/test/supported_types/type_test.string_test.dart b/json_serializable/test/supported_types/type_test.string_test.dart
index 41af3050c..845762b19 100644
--- a/json_serializable/test/supported_types/type_test.string_test.dart
+++ b/json_serializable/test/supported_types/type_test.string_test.dart
@@ -2,8 +2,6 @@
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
-// @dart=2.12
-
 // ignore_for_file: prefer_const_declarations
 
 import 'dart:convert';
diff --git a/json_serializable/test/supported_types/type_test.uri_test.dart b/json_serializable/test/supported_types/type_test.uri_test.dart
index b32b892cc..ff0710b38 100644
--- a/json_serializable/test/supported_types/type_test.uri_test.dart
+++ b/json_serializable/test/supported_types/type_test.uri_test.dart
@@ -2,8 +2,6 @@
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
-// @dart=2.12
-
 // ignore_for_file: prefer_const_declarations
 
 import 'dart:convert';
diff --git a/json_serializable/test/test_sources/test_sources.dart b/json_serializable/test/test_sources/test_sources.dart
index e14ad6205..b1182cb06 100644
--- a/json_serializable/test/test_sources/test_sources.dart
+++ b/json_serializable/test/test_sources/test_sources.dart
@@ -1,5 +1,3 @@
-// @dart=2.12
-
 import 'package:json_annotation/json_annotation.dart';
 
 @JsonSerializable()
diff --git a/json_serializable/test/test_utils.dart b/json_serializable/test/test_utils.dart
index 324cdb27a..6bb67f98a 100644
--- a/json_serializable/test/test_utils.dart
+++ b/json_serializable/test/test_utils.dart
@@ -2,8 +2,6 @@
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
-// @dart=2.12
-
 import 'dart:convert';
 
 import 'package:stack_trace/stack_trace.dart';
diff --git a/json_serializable/tool/doc_builder.dart b/json_serializable/tool/doc_builder.dart
index 0dd737305..267690f7a 100644
--- a/json_serializable/tool/doc_builder.dart
+++ b/json_serializable/tool/doc_builder.dart
@@ -45,7 +45,7 @@ class _DocBuilder extends Builder {
 
     for (var className in _annotationClasses) {
       for (var fe in lib
-          .findType(className)
+          .findType(className)!
           .fields
           .where((fe) => !fe.isStatic && !fe.hasDeprecated)) {
         descriptionMap[fe.name] =
@@ -121,9 +121,9 @@ String _link(String version, String owner, String name) =>
     'json_annotation/$owner/$name.html';
 
 class _FieldInfo implements Comparable<_FieldInfo> {
-  final FieldElement _keyField, _classField;
+  final FieldElement? _keyField, _classField;
 
-  String get name => _keyField?.name ?? _classField.name;
+  String get name => _keyField?.name ?? _classField!.name;
 
   String get classAnnotationName {
     if (_classField == null) {
@@ -144,15 +144,15 @@ class _FieldInfo implements Comparable<_FieldInfo> {
       return '';
     }
 
-    return snakeCase(_classField.name);
+    return snakeCase(_classField!.name);
   }
 
   _FieldInfo(this._keyField, this._classField);
 
-  static _FieldInfo update(FieldElement field, _FieldInfo existing) {
+  static _FieldInfo update(FieldElement field, _FieldInfo? existing) {
     final parent = field.enclosingElement.name;
 
-    FieldElement keyField, classField;
+    FieldElement? keyField, classField;
     switch (parent) {
       case _jsonSerializable:
         classField = field;
diff --git a/json_serializable/tool/shared.dart b/json_serializable/tool/shared.dart
index 3e12e469e..f1e0b9914 100644
--- a/json_serializable/tool/shared.dart
+++ b/json_serializable/tool/shared.dart
@@ -21,7 +21,7 @@ Builder validate(String builderName, Builder builder) {
 
   final extensions = Set<String>.from(buildYaml['.dart'] as YamlList);
 
-  final codedExtensions = builder.buildExtensions['.dart'].toSet();
+  final codedExtensions = builder.buildExtensions['.dart']!.toSet();
 
   final tooMany = extensions.difference(codedExtensions);
   if (tooMany.isNotEmpty) {
diff --git a/json_serializable/tool/test_builder.dart b/json_serializable/tool/test_builder.dart
index 52c4a3b70..4282e43e1 100644
--- a/json_serializable/tool/test_builder.dart
+++ b/json_serializable/tool/test_builder.dart
@@ -27,7 +27,7 @@ class _TestBuilder implements Builder {
     final factories =
         SplayTreeMap.from({'$_kitchenSinkBaseName.dart': 'normal'});
 
-    for (var config in _fileConfigurationMap[baseName]) {
+    for (var config in _fileConfigurationMap[baseName]!) {
       final extension = _configToExtension(config);
       final newId = buildStep.inputId.changeExtension(extension);
 
@@ -65,7 +65,7 @@ class _TestBuilder implements Builder {
       final lines = <String>[
         r'''
 // GENERATED CODE - DO NOT MODIFY BY HAND
-// @dart=2.12
+
 ''',
         ...factories.entries.map((e) => "import '${e.key}' as ${e.value};"),
         'const factories = [',
@@ -146,7 +146,7 @@ Iterable<Replacement> _optionReplacement(
 
   if (baseName == _kitchenSinkBaseName &&
       _kitchenSinkReplacements.containsKey(optionKey)) {
-    yield* _kitchenSinkReplacements[optionKey];
+    yield* _kitchenSinkReplacements[optionKey]!;
   }
 }
 
diff --git a/json_serializable/tool/test_type_data.dart b/json_serializable/tool/test_type_data.dart
index fbe201ced..bd310a751 100644
--- a/json_serializable/tool/test_type_data.dart
+++ b/json_serializable/tool/test_type_data.dart
@@ -1,5 +1,3 @@
-import 'package:meta/meta.dart';
-
 import 'shared.dart';
 
 const customEnumType = 'EnumType';
@@ -8,15 +6,15 @@ const _annotationImport =
     "import 'package:json_annotation/json_annotation.dart';";
 
 class TestTypeData {
-  final String defaultExpression;
-  final String jsonExpression;
-  final String altJsonExpression;
+  final String? defaultExpression;
+  final String? jsonExpression;
+  final String? altJsonExpression;
   final Set<String> genericArgs;
 
   const TestTypeData({
     this.defaultExpression,
-    String jsonExpression,
-    @required String altJsonExpression,
+    String? jsonExpression,
+    required String? altJsonExpression,
     this.genericArgs = const {},
   })  : jsonExpression = jsonExpression ?? defaultExpression,
         altJsonExpression =
@@ -110,7 +108,7 @@ class TestTypeData {
     final defaultReplacement = defaultNotSupported
         ? ''
         : _defaultSource
-            .replaceFirst('42', defaultExpression)
+            .replaceFirst('42', defaultExpression!)
             .replaceFirst('dynamic', type);
 
     yield Replacement(