Feat: Add brightness export from widgets.dart (#169410)

Feat: Add brightness export from widgets.dart
fixes: #169294 

## Pre-launch Checklist

- [x] I read the [Contributor Guide] and followed the process outlined
there for submitting PRs.
- [x] I read the [Tree Hygiene] wiki page, which explains my
responsibilities.
- [x] I read and followed the [Flutter Style Guide], including [Features
we expect every widget to implement].
- [x] I signed the [CLA].
- [x] I listed at least one issue that this PR fixes in the description
above.
- [x] I updated/added relevant documentation (doc comments with `///`).
- [ ] I added new tests to check the change I am making, or this PR is
[test-exempt].
- [x] I followed the [breaking change policy] and added [Data Driven
Fixes] where supported.
- [x] All existing and new tests are passing.

Author: rkishan516

packages/flutter/lib/src/material/desktop_text_selection_toolbar_button.dart

@@ -2,7 +2,6 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
-import 'package:flutter/services.dart';
 import 'package:flutter/widgets.dart';
 
 import 'colors.dart';

packages/flutter/lib/src/material/material_button.dart

@@ -12,8 +12,6 @@
 /// @docImport 'text_theme.dart';
 library;
 
-import 'dart:ui';
-
 import 'package:flutter/rendering.dart';
 import 'package:flutter/widgets.dart';
 

packages/flutter/lib/widgets.dart

@@ -15,7 +15,7 @@ library widgets;
 export 'package:characters/characters.dart';
 export 'package:vector_math/vector_math_64.dart' show Matrix4;
 
-export 'foundation.dart' show UniqueKey;
+export 'foundation.dart' show Brightness, UniqueKey;
 export 'rendering.dart' show TextSelectionHandleType;
 export 'src/widgets/actions.dart';
 export 'src/widgets/adapter.dart';

packages/flutter_tools/templates/widget_preview_scaffold/lib/src/widget_preview.dart.tmpl

@@ -2,8 +2,6 @@
 // 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/foundation.dart';
 import 'package:flutter/widget_previews.dart';
 import 'package:flutter/widgets.dart';

packages/flutter_tools/test/widget_preview_scaffold.shard/widget_preview_scaffold/lib/src/widget_preview.dart

@@ -2,8 +2,6 @@
 // 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/foundation.dart';
 import 'package:flutter/widget_previews.dart';
 import 'package:flutter/widgets.dart';