enable lint prefer_void_to_null (#858)

analysis_options.yaml

@@ -112,6 +112,7 @@ linter:
     # - prefer_interpolation_to_compose_strings # not yet tested
     - prefer_is_empty
     - prefer_is_not_empty
+    - prefer_void_to_null
     # - recursive_getters # https://github.com/dart-lang/linter/issues/452
     - slash_for_doc_comments
     - sort_constructors_first

packages/android_alarm_manager/example/lib/main.dart

@@ -12,7 +12,7 @@ import 'package:flutter/widgets.dart';
 final FirebaseAuth firebaseAuth = FirebaseAuth.instance;
 FirebaseUser firebaseUser;
 
-Future<Null> ensureFirebaseUser() async {
+Future<void> ensureFirebaseUser() async {
   if (firebaseUser == null) {
     firebaseUser = await firebaseAuth.currentUser();
     if (firebaseUser == null) {
@@ -67,7 +67,7 @@ void printOneShot() {
   printHelloMessage("Hello, once!");
 }
 
-Future<Null> main() async {
+Future<void> main() async {
   final int helloAlarmID = 0;
   final int goodbyeAlarmID = 1;
   final int oneShotID = 2;

packages/android_intent/lib/android_intent.dart

@@ -42,7 +42,7 @@ class AndroidIntent {
   ///
   /// This works only on Android platforms. Please guard the call so that your
   /// iOS app does not crash. Checked mode will throw an assert exception.
-  Future<Null> launch() async {
+  Future<void> launch() async {
     assert(_platform.isAndroid);
     final Map<String, dynamic> args = <String, dynamic>{'action': action};
     if (category != null) {

packages/battery/example/lib/main.dart

@@ -63,7 +63,7 @@ class _MyHomePageState extends State<MyHomePage> {
         child: const Icon(Icons.battery_unknown),
         onPressed: () async {
           final int batteryLevel = await _battery.batteryLevel;
-          showDialog<Null>(
+          showDialog<void>(
             context: context,
             builder: (_) => AlertDialog(
                   content: Text('Battery: $batteryLevel%'),

packages/camera/README.md

@@ -48,7 +48,7 @@ import 'package:camera/camera.dart';
 
 List<CameraDescription> cameras;
 
-Future<Null> main() async {
+Future<void> main() async {
   cameras = await availableCameras();
   runApp(CameraApp());
 }

packages/camera/example/lib/main.dart

@@ -356,7 +356,7 @@ class CameraApp extends StatelessWidget {
 
 List<CameraDescription> cameras;
 
-Future<Null> main() async {
+Future<void> main() async {
   // Fetch the available cameras before initializing the app.
   try {
     cameras = await availableCameras();

packages/camera/lib/camera.dart

@@ -185,17 +185,17 @@ class CameraController extends ValueNotifier<CameraValue> {
   int _textureId;
   bool _isDisposed = false;
   StreamSubscription<dynamic> _eventSubscription;
-  Completer<Null> _creatingCompleter;
+  Completer<void> _creatingCompleter;
 
   /// Initializes the camera on the device.
   ///
   /// Throws a [CameraException] if the initialization fails.
-  Future<Null> initialize() async {
+  Future<void> initialize() async {
     if (_isDisposed) {
-      return Future<Null>.value(null);
+      return Future<void>.value();
     }
     try {
-      _creatingCompleter = Completer<Null>();
+      _creatingCompleter = Completer<void>();
       final Map<dynamic, dynamic> reply = await _channel.invokeMethod(
         'initialize',
         <String, dynamic>{
@@ -218,7 +218,7 @@ class CameraController extends ValueNotifier<CameraValue> {
         EventChannel('flutter.io/cameraPlugin/cameraEvents$_textureId')
             .receiveBroadcastStream()
             .listen(_listener);
-    _creatingCompleter.complete(null);
+    _creatingCompleter.complete();
     return _creatingCompleter.future;
   }
 
@@ -250,7 +250,7 @@ class CameraController extends ValueNotifier<CameraValue> {
   /// The file can be read as this function returns.
   ///
   /// Throws a [CameraException] if the capture fails.
-  Future<Null> takePicture(String path) async {
+  Future<void> takePicture(String path) async {
     if (!value.isInitialized || _isDisposed) {
       throw CameraException(
         'Uninitialized CameraController.',
@@ -286,7 +286,7 @@ class CameraController extends ValueNotifier<CameraValue> {
   /// The file can be read as soon as [stopVideoRecording] returns.
   ///
   /// Throws a [CameraException] if the capture fails.
-  Future<Null> startVideoRecording(String filePath) async {
+  Future<void> startVideoRecording(String filePath) async {
     if (!value.isInitialized || _isDisposed) {
       throw CameraException(
         'Uninitialized CameraController',
@@ -311,7 +311,7 @@ class CameraController extends ValueNotifier<CameraValue> {
   }
 
   /// Stop recording.
-  Future<Null> stopVideoRecording() async {
+  Future<void> stopVideoRecording() async {
     if (!value.isInitialized || _isDisposed) {
       throw CameraException(
         'Uninitialized CameraController',
@@ -337,22 +337,19 @@ class CameraController extends ValueNotifier<CameraValue> {
 
   /// Releases the resources of this camera.
   @override
-  Future<Null> dispose() async {
+  Future<void> dispose() async {
     if (_isDisposed) {
-      return Future<Null>.value(null);
+      return;
     }
     _isDisposed = true;
     super.dispose();
-    if (_creatingCompleter == null) {
-      return Future<Null>.value(null);
-    } else {
-      return _creatingCompleter.future.then((_) async {
-        await _channel.invokeMethod(
-          'dispose',
-          <String, dynamic>{'textureId': _textureId},
-        );
-        await _eventSubscription?.cancel();
-      });
+    if (_creatingCompleter != null) {
+      await _creatingCompleter.future;
+      await _channel.invokeMethod(
+        'dispose',
+        <String, dynamic>{'textureId': _textureId},
+      );
+      await _eventSubscription?.cancel();
     }
   }
 }

packages/cloud_firestore/example/lib/main.dart

@@ -57,7 +57,7 @@ class MyHomePage extends StatelessWidget {
   final Firestore firestore;
   CollectionReference get messages => firestore.collection('messages');
 
-  Future<Null> _addMessage() async {
+  Future<void> _addMessage() async {
     await messages.add(<String, dynamic>{
       'message': 'Hello world!',
       'created_at': FieldValue.serverTimestamp(),

packages/cloud_firestore/lib/src/write_batch.dart

@@ -25,11 +25,11 @@ class WriteBatch {
   /// Commits all of the writes in this write batch as a single atomic unit.
   ///
   /// Calling this method prevents any future operations from being added.
-  Future<Null> commit() async {
+  Future<void> commit() async {
     if (!_committed) {
       _committed = true;
       await Future.wait<dynamic>(_actions);
-      return await Firestore.channel.invokeMethod(
+      await Firestore.channel.invokeMethod(
           'WriteBatch#commit', <String, dynamic>{'handle': await _handle});
     } else {
       throw StateError("This batch has already been committed.");

packages/cloud_firestore/test/cloud_firestore_test.dart

@@ -279,7 +279,7 @@ void main() {
         expect(document.reference.path, equals('foo/0'));
         expect(document.data, equals(kMockDocumentSnapshotData));
         // Flush the async removeListener call
-        await Future<Null>.delayed(Duration.zero);
+        await Future<void>.delayed(Duration.zero);
         expect(log, <Matcher>[
           isMethodCall(
             'Query#addSnapshotListener',
@@ -305,7 +305,7 @@ void main() {
                 .snapshots()
                 .listen((QuerySnapshot querySnapshot) {});
         subscription.cancel();
-        await Future<Null>.delayed(Duration.zero);
+        await Future<void>.delayed(Duration.zero);
         expect(
           log,
           equals(<Matcher>[
@@ -336,7 +336,7 @@ void main() {
                 .snapshots()
                 .listen((QuerySnapshot querySnapshot) {});
         subscription.cancel();
-        await Future<Null>.delayed(Duration.zero);
+        await Future<void>.delayed(Duration.zero);
         expect(
           log,
           equals(<Matcher>[
@@ -367,7 +367,7 @@ void main() {
                 .snapshots()
                 .listen((QuerySnapshot querySnapshot) {});
         subscription.cancel();
-        await Future<Null>.delayed(Duration.zero);
+        await Future<void>.delayed(Duration.zero);
         expect(
           log,
           equals(<Matcher>[
@@ -401,7 +401,7 @@ void main() {
         expect(snapshot.reference.path, equals('path/to/foo'));
         expect(snapshot.data, equals(kMockDocumentSnapshotData));
         // Flush the async removeListener call
-        await Future<Null>.delayed(Duration.zero);
+        await Future<void>.delayed(Duration.zero);
         expect(
           log,
           <Matcher>[

packages/connectivity/example/lib/main.dart

@@ -57,7 +57,7 @@ class _MyHomePageState extends State<MyHomePage> {
   }
 
   // Platform messages are asynchronous, so we initialize in an async method.
-  Future<Null> initConnectivity() async {
+  Future<void> initConnectivity() async {
     String connectionStatus;
     // Platform messages may fail, so we use a try/catch PlatformException.
     try {

packages/device_info/example/lib/main.dart

@@ -33,7 +33,7 @@ class _MyAppState extends State<MyApp> {
     initPlatformState();
   }
 
-  Future<Null> initPlatformState() async {
+  Future<void> initPlatformState() async {
     Map<String, dynamic> deviceData;
 
     try {

packages/firebase_admob/lib/firebase_admob.dart

@@ -501,7 +501,7 @@ class FirebaseAdMob {
       }
     }
 
-    return Future<Null>(null);
+    return Future<dynamic>.value(null);
   }
 }
 

packages/firebase_analytics/example/lib/main.dart

@@ -61,7 +61,7 @@ class _MyHomePageState extends State<MyHomePage> {
     });
   }
 
-  Future<Null> _sendAnalyticsEvent() async {
+  Future<void> _sendAnalyticsEvent() async {
     await analytics.logEvent(
       name: 'test_event',
       parameters: <String, dynamic>{
@@ -75,41 +75,41 @@ class _MyHomePageState extends State<MyHomePage> {
     setMessage('logEvent succeeded');
   }
 
-  Future<Null> _testSetUserId() async {
+  Future<void> _testSetUserId() async {
     await analytics.setUserId('some-user');
     setMessage('setUserId succeeded');
   }
 
-  Future<Null> _testSetCurrentScreen() async {
+  Future<void> _testSetCurrentScreen() async {
     await analytics.setCurrentScreen(
       screenName: 'Analytics Demo',
       screenClassOverride: 'AnalyticsDemo',
     );
     setMessage('setCurrentScreen succeeded');
   }
 
-  Future<Null> _testSetAnalyticsCollectionEnabled() async {
+  Future<void> _testSetAnalyticsCollectionEnabled() async {
     await analytics.android?.setAnalyticsCollectionEnabled(false);
     await analytics.android?.setAnalyticsCollectionEnabled(true);
     setMessage('setAnalyticsCollectionEnabled succeeded');
   }
 
-  Future<Null> _testSetMinimumSessionDuration() async {
+  Future<void> _testSetMinimumSessionDuration() async {
     await analytics.android?.setMinimumSessionDuration(20000);
     setMessage('setMinimumSessionDuration succeeded');
   }
 
-  Future<Null> _testSetSessionTimeoutDuration() async {
+  Future<void> _testSetSessionTimeoutDuration() async {
     await analytics.android?.setSessionTimeoutDuration(2000000);
     setMessage('setSessionTimeoutDuration succeeded');
   }
 
-  Future<Null> _testSetUserProperty() async {
+  Future<void> _testSetUserProperty() async {
     await analytics.setUserProperty(name: 'regular', value: 'indeed');
     setMessage('setUserProperty succeeded');
   }
 
-  Future<Null> _testAllEventTypes() async {
+  Future<void> _testAllEventTypes() async {
     await analytics.logAddPaymentInfo();
     await analytics.logAddToCart(
       currency: 'USD',