Add WidgetStateProperty example and tests for it. (flutter#155315)

This PR contributes to flutter#155313

### Description
- Adds example for `WidgetStateProperty`
- Adds tests for `examples/api/lib/widgets/widget_state/widget_state_property.0.dart`

Author: ksokolovskyi

examples/api/lib/widgets/widget_state/widget_state_property.0.dart

@@ -0,0 +1,48 @@
+// Copyright 2014 The Flutter 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';
+
+/// Flutter code sample for [WidgetStateProperty].
+
+void main() {
+  runApp(const WidgetStatePropertyExampleApp());
+}
+
+class WidgetStatePropertyExampleApp extends StatelessWidget {
+  const WidgetStatePropertyExampleApp({super.key});
+
+  @override
+  Widget build(BuildContext context) {
+    return MaterialApp(
+      home: Scaffold(
+        appBar: AppBar(title: const Text('WidgetStateProperty Sample')),
+        body: const Center(
+          child: WidgetStatePropertyExample(),
+        ),
+      ),
+    );
+  }
+}
+
+class WidgetStatePropertyExample extends StatelessWidget {
+  const WidgetStatePropertyExample({super.key});
+
+  @override
+  Widget build(BuildContext context) {
+    return TextButton(
+      style: ButtonStyle(
+        foregroundColor: WidgetStateProperty<Color>.fromMap(
+          <WidgetStatesConstraint, Color>{
+            WidgetState.focused: Colors.blueAccent,
+            WidgetState.pressed | WidgetState.hovered: Colors.blue,
+            WidgetState.any: Colors.red,
+          },
+        ),
+      ),
+      onPressed: () {},
+      child: const Text('TextButton'),
+    );
+  }
+}

examples/api/test/widgets/widget_state/widget_state_property.0_test.dart

@@ -0,0 +1,74 @@
+// Copyright 2014 The Flutter 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/material.dart';
+import 'package:flutter_api_samples/widgets/widget_state/widget_state_property.0.dart'
+    as example;
+import 'package:flutter_test/flutter_test.dart';
+
+void main() {
+  Color? getTextColor(WidgetTester tester) {
+    final BuildContext context = tester.element(find.text('TextButton'));
+    final TextStyle textStyle = DefaultTextStyle.of(context).style;
+
+    return textStyle.color;
+  }
+
+  testWidgets('Displays red colored text by default', (WidgetTester tester) async {
+    await tester.pumpWidget(
+      const example.WidgetStatePropertyExampleApp(),
+    );
+
+    expect(getTextColor(tester), Colors.red);
+  });
+
+  testWidgets('Displays blue colored text when button is hovered', (WidgetTester tester) async {
+    await tester.pumpWidget(
+      const example.WidgetStatePropertyExampleApp(),
+    );
+
+    expect(getTextColor(tester), Colors.red);
+
+    // Hover over the TextButton.
+    final TestGesture gesture = await tester.createGesture(kind: PointerDeviceKind.mouse);
+    await gesture.moveTo(tester.getCenter(find.byType(TextButton)));
+
+    await tester.pumpAndSettle();
+
+    expect(getTextColor(tester), Colors.blue);
+  });
+
+  testWidgets('Displays blue colored text when button is pressed', (WidgetTester tester) async {
+    await tester.pumpWidget(
+      const example.WidgetStatePropertyExampleApp(),
+    );
+
+    expect(getTextColor(tester), Colors.red);
+
+    // Press on the TextButton.
+    final TestGesture gesture = await tester.createGesture();
+    await gesture.down(tester.getCenter(find.byType(TextButton)));
+
+    await tester.pumpAndSettle();
+
+    expect(getTextColor(tester), Colors.blue);
+  });
+
+  testWidgets('Displays blue colored text when button is focused', (WidgetTester tester) async {
+    await tester.pumpWidget(
+      const example.WidgetStatePropertyExampleApp(),
+    );
+
+    expect(getTextColor(tester), Colors.red);
+
+    // Focus on the TextButton.
+    FocusScope.of(tester.element(find.byType(TextButton))).nextFocus();
+
+    await tester.pumpAndSettle();
+
+    expect(getTextColor(tester), Colors.blueAccent);
+  });
+}

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

@@ -765,6 +765,16 @@ class _WidgetTextStyleMapper extends WidgetStateTextStyle {
 /// of their current material state and [resolve] the button style's
 /// material state properties when their value is needed.
 ///
+/// {@tool dartpad}
+/// This example shows how the default text and icon color
+/// (the "foreground color") of a [TextButton] can be overridden with a
+/// [WidgetStateProperty]. In this example, the button's text color will be
+/// `Colors.blue` when the button is being pressed, hovered, or focused.
+/// Otherwise, the text color will be `Colors.red`.
+///
+/// ** See code in examples/api/lib/widgets/widget_state/widget_state_property.0.dart **
+/// {@end-tool}
+///
 /// See also:
 ///
 ///  * [MaterialStateProperty], the Material specific version of