Support unnamed libraries

lib/src/rules/library_names.dart

@@ -57,8 +57,9 @@ class _Visitor extends SimpleAstVisitor<void> {
 
   @override
   void visitLibraryDirective(LibraryDirective node) {
-    if (!isLowerCaseUnderScoreWithDots(node.name2.toString())) {
-      rule.reportLint(node.name2);
+    var name = node.name2;
+    if (name != null && !isLowerCaseUnderScoreWithDots(name.toString())) {
+      rule.reportLint(name);
     }
   }
 }

test/rules/all.dart

@@ -33,6 +33,7 @@ import 'discarded_futures_test.dart' as discarded_futures;
 import 'file_names_test.dart' as file_names;
 import 'flutter_style_todos_test.dart' as flutter_style_todos;
 import 'hash_and_equals_test.dart' as hash_and_equals;
+import 'library_names_test.dart' as library_names;
 import 'library_private_types_in_public_api_test.dart'
     as library_private_types_in_public_api;
 import 'literal_only_boolean_expressions_test.dart'
@@ -106,6 +107,7 @@ void main() {
   file_names.main();
   flutter_style_todos.main();
   hash_and_equals.main();
+  library_names.main();
   library_private_types_in_public_api.main();
   literal_only_boolean_expressions.main();
   missing_whitespace_between_adjacent_strings.main();

test/rules/library_names_test.dart

@@ -0,0 +1,55 @@
+// Copyright (c) 2022, the Dart project authors. Please see the AUTHORS file
+// 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.
+
+import 'package:test_reflective_loader/test_reflective_loader.dart';
+
+import '../rule_test_support.dart';
+
+main() {
+  defineReflectiveSuite(() {
+    defineReflectiveTests(LibraryNamesTest);
+  });
+}
+
+@reflectiveTest
+class LibraryNamesTest extends LintRuleTest {
+  @override
+  List<String> get experiments => ['unnamed-libraries'];
+
+  @override
+  String get lintRule => 'library_names';
+
+  test_libraryWithoutName() async {
+    await assertNoDiagnostics('''
+library;
+''');
+  }
+
+  test_lowercase() async {
+    await assertNoDiagnostics('''
+library foo;
+''');
+  }
+
+  test_noLibrary() async {
+    await assertNoDiagnostics('''
+''');
+  }
+
+  test_titlecase() async {
+    await assertDiagnostics('''
+library Foo;
+''', [
+      lint(8, 3),
+    ]);
+  }
+
+  test_uppercaseInDots() async {
+    await assertDiagnostics('''
+library one.Two.three;
+''', [
+      lint(8, 13),
+    ]);
+  }
+}