Add LICENSE test to presubmit checks (#28369)

## Description

Also update the existing dart files with missing licenses.

Without the fix, we'll emit the following error message
```
License headers cannot be found at the beginning of the following files.

/usr/local/google/home/liyuqian/flutter/flutter/packages/flutter/lib/src/animation/tween_sequence.dart
/usr/local/google/home/liyuqian/flutter/flutter/packages/flutter/test/material/raw_material_button_test.dart
/usr/local/google/home/liyuqian/flutter/flutter/packages/flutter/test/widgets/async_lifecycle_test.dart
/usr/local/google/home/liyuqian/flutter/flutter/packages/flutter/test/widgets/sliver_constraints_test.dart
/usr/local/google/home/liyuqian/flutter/flutter/packages/flutter/test/widgets/app_test.dart
/usr/local/google/home/liyuqian/flutter/flutter/packages/flutter/test/widgets/test_border.dart
/usr/local/google/home/liyuqian/flutter/flutter/packages/flutter/test/widgets/physical_model_test.dart
/usr/local/google/home/liyuqian/flutter/flutter/packages/flutter/test/widgets/inherited_model.dart
/usr/local/google/home/liyuqian/flutter/flutter/packages/flutter_tools/lib/src/base/user_messages.dart
/usr/local/google/home/liyuqian/flutter/flutter/packages/flutter_tools/test/src/pubspec_schema.dart
/usr/local/google/home/liyuqian/flutter/flutter/packages/flutter_tools/test/ios/simulators_test.dart
━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━
```

## Related Issues

Fixes https://github.com/flutter/flutter/issues/28368

## Checklist

Before you create this PR confirm that it meets all requirements listed below by checking the relevant checkboxes (`[x]`). This will ensure a smooth and quick review process.

- [x] I read the [Contributor Guide] and followed the process outlined there for submitting PRs.
- [x] My PR includes tests for *all* changed/updated/fixed behaviors (See [Test Coverage]).
- [x] All existing and new tests are passing.
- [x] I updated/added relevant documentation (doc comments with `///`).
- [x] The analyzer (`flutter analyze --flutter-repo`) does not report any problems on my PR.
- [x] I read and followed the [Flutter Style Guide], including [Features we expect every widget to implement].
- [x] I signed the [CLA].
- [x] I am willing to follow-up on review comments in a timely manner.

## Breaking Change

Does your PR require Flutter developers to manually update their apps to accommodate your change?

- [ ] Yes, this is a breaking change (Please read [Handling breaking changes]).
- [x] No, this is *not* a breaking change.

<!-- Links -->
[issue database]: https://github.com/flutter/flutter/issues
[Contributor Guide]: https://github.com/flutter/flutter/wiki/Tree-hygiene#overview
[Test Coverage]: https://github.com/flutter/flutter/wiki/Test-coverage-for-package%3Aflutter
[Flutter Style Guide]: https://github.com/flutter/flutter/wiki/Style-guide-for-Flutter-repo
[Features we expect every widget to implement]: https://github.com/flutter/flutter/wiki/Style-guide-for-Flutter-repo#features-we-expect-every-widget-to-implement
[CLA]: https://cla.developers.google.com/
[Handling breaking changes]: https://github.com/flutter/flutter/wiki/Tree-hygiene#handling-breaking-changes

dev/bots/analyze.dart

@@ -30,6 +30,7 @@ Future<void> main(List<String> args) async {
     print('The analyze.dart script must be run with --enable-asserts.');
     exit(1);
   }
+  await _verifyNoMissingLicense(flutterRoot);
   await _verifyNoTestImports(flutterRoot);
   await _verifyNoTestPackageImports(flutterRoot);
   await _verifyGeneratedPluginRegistrants(flutterRoot);
@@ -472,6 +473,30 @@ Future<void> _verifyNoBadImportsInFlutterTools(String workingDirectory) async {
   }
 }
 
+Future<void> _verifyNoMissingLicense(String workingDirectory) async {
+  final List<String> errors = <String>[];
+  for (FileSystemEntity entity in Directory(path.join(workingDirectory, 'packages'))
+      .listSync(recursive: true)
+      .where((FileSystemEntity entity) => entity is File && path.extension(entity.path) == '.dart')) {
+    final File file = entity;
+    bool hasLicense = false;
+    final List<String> lines = file.readAsLinesSync();
+    if (lines.isNotEmpty)
+      hasLicense = lines.first.startsWith(RegExp(r'// Copyright \d{4}'));
+    if (!hasLicense)
+      errors.add(file.path);
+  }
+  // Fail if any errors
+  if (errors.isNotEmpty) {
+    print('$redLine');
+    final String s = errors.length == 1 ? '' : 's';
+    print('${bold}License headers cannot be found at the beginning of the following file$s.$reset\n');
+    print(errors.join('\n'));
+    print('$redLine\n');
+    exit(1);
+  }
+}
+
 final RegExp _testImportPattern = RegExp(r'''import (['"])([^'"]+_test\.dart)\1''');
 const Set<String> _exemptTestImports = <String>{
   'package:flutter_test/flutter_test.dart',

packages/flutter/lib/src/animation/tween_sequence.dart

+// Copyright 2018 The Chromium Authors. 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:flutter/foundation.dart';
 
 import 'animation.dart';

packages/flutter/lib/src/widgets/scroll_configuration.dart

-/// Copyright 2016 The Chromium Authors. All rights reserved.
+// Copyright 2016 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 

packages/flutter/test/material/mergeable_material_test.dart

-  // Copyright 2016 The Chromium Authors. All rights reserved.
+// Copyright 2016 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 

packages/flutter/test/material/raw_material_button_test.dart

+// Copyright 2018 The Chromium Authors. 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:flutter/material.dart';
 import 'package:flutter/rendering.dart';
 import 'package:flutter_test/flutter_test.dart';

packages/flutter/test/material/tooltip_test.dart

-import 'dart:ui';
-
 // Copyright 2015 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
+import 'dart:ui';
+
 import 'package:flutter/services.dart';
 import 'package:flutter_test/flutter_test.dart';
 import 'package:flutter/gestures.dart';

packages/flutter/test/rendering/editable_test.dart

- // Copyright 2017 The Chromium Authors. All rights reserved.
+// Copyright 2017 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 

packages/flutter/test/widgets/app_test.dart

+// Copyright 2018 The Chromium Authors. 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:flutter/widgets.dart';
 import 'package:flutter_test/flutter_test.dart';
 

packages/flutter/test/widgets/async_lifecycle_test.dart

+// Copyright 2018 The Chromium Authors. 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:flutter/widgets.dart';
 import 'package:flutter_test/flutter_test.dart';
 

packages/flutter/test/widgets/physical_model_test.dart

+// Copyright 2018 The Chromium Authors. 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:flutter/material.dart';
 import 'package:flutter/rendering.dart';
 import 'package:flutter/widgets.dart';

packages/flutter/test/widgets/sliver_constraints_test.dart

+// Copyright 2019 The Chromium Authors. 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:flutter/material.dart';
 import 'package:flutter/rendering.dart';
 import 'package:flutter/widgets.dart';

packages/flutter/test/widgets/test_border.dart

+// Copyright 2019 The Chromium Authors. 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:flutter/material.dart';
 
 typedef Logger = void Function(String caller);

packages/flutter_tools/lib/src/base/user_messages.dart

+// Copyright 2018 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
 import 'context.dart';
 
 UserMessages get userMessages => context[UserMessages];

packages/flutter_tools/lib/src/build_runner/build_script_generator.dart

-// Copyright 201 The Chromium Authors. All rights reserved.
+// Copyright 2019 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 

packages/flutter_tools/test/ios/simulators_test.dart

+// Copyright 2017 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
 import 'dart:async';
 import 'dart:io' show ProcessResult, Process;
 

packages/flutter_tools/test/protocol_discovery_test.dart

-
 // Copyright 2016 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.

packages/flutter_tools/test/src/pubspec_schema.dart

+// Copyright 2018 The Chromium Authors. 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:file/file.dart';
 import 'package:flutter_tools/src/base/file_system.dart';
 import 'package:flutter_tools/src/flutter_manifest.dart';