Add WidgetStateBorderSide example and tests for it. (flutter#155559)

Fixes flutter#155557

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

Author: ksokolovskyi

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

@@ -0,0 +1,60 @@
+// 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 [WidgetStateBorderSide].
+
+void main() {
+  runApp(const WidgetStateBorderSideExampleApp());
+}
+
+class WidgetStateBorderSideExampleApp extends StatelessWidget {
+  const WidgetStateBorderSideExampleApp({super.key});
+
+  @override
+  Widget build(BuildContext context) {
+    return MaterialApp(
+      home: Scaffold(
+        appBar: AppBar(title: const Text('WidgetStateBorderSide Sample')),
+        body: const Center(
+          child: WidgetStateBorderSideExample(),
+        ),
+      ),
+    );
+  }
+}
+
+class WidgetStateBorderSideExample extends StatefulWidget {
+  const WidgetStateBorderSideExample({super.key});
+
+  @override
+  State<WidgetStateBorderSideExample> createState() => _WidgetStateBorderSideExampleState();
+}
+
+class _WidgetStateBorderSideExampleState extends State<WidgetStateBorderSideExample> {
+  bool _isSelected = true;
+
+  @override
+  Widget build(BuildContext context) {
+    return FilterChip(
+      label: const Text('Select chip'),
+      selected: _isSelected,
+      onSelected: (bool value) {
+        setState(() {
+          _isSelected = value;
+        });
+      },
+      side: const WidgetStateBorderSide.fromMap(
+        <WidgetStatesConstraint, BorderSide?>{
+          WidgetState.pressed:  BorderSide(color: Colors.green),
+          WidgetState.hovered:  BorderSide(color: Colors.blue),
+          WidgetState.selected: BorderSide(color: Colors.red),
+          // Resolves to null if no keys match, deferring to the default value
+          // of the theme or widget.
+        },
+      ),
+    );
+  }
+}

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

@@ -0,0 +1,76 @@
+// 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_border_side.0.dart'
+    as example;
+import 'package:flutter_test/flutter_test.dart';
+
+void main() {
+  Finder findByBorderColor(Color color) {
+    return find.byWidgetPredicate((Widget widget) {
+      if (widget is! Material) {
+        return false;
+      }
+
+      final ShapeBorder? shape = widget.shape;
+      return shape is OutlinedBorder && shape.side.color == color;
+    });
+  }
+
+  testWidgets('FilterChip displays the blue colored border when hovered', (WidgetTester tester) async {
+    await tester.pumpWidget(
+      const example.WidgetStateBorderSideExampleApp(),
+    );
+
+    // Hover over the FilterChip.
+    final TestGesture gesture = await tester.createGesture(kind: PointerDeviceKind.mouse);
+    await gesture.moveTo(tester.getCenter(find.byType(FilterChip)));
+
+    await tester.pumpAndSettle();
+
+    expect(findByBorderColor(Colors.blue), findsOneWidget);
+  });
+
+  testWidgets('FilterChip displays the green colored border when pressed', (WidgetTester tester) async {
+    await tester.pumpWidget(
+      const example.WidgetStateBorderSideExampleApp(),
+    );
+
+    // Press on the FilterChip.
+    final TestGesture gesture = await tester.createGesture();
+    await gesture.down(tester.getCenter(find.byType(FilterChip)));
+
+    await tester.pumpAndSettle();
+
+    expect(findByBorderColor(Colors.green), findsOneWidget);
+  });
+
+  testWidgets('FilterChip displays the red colored border when selected', (WidgetTester tester) async {
+    await tester.pumpWidget(
+      const example.WidgetStateBorderSideExampleApp(),
+    );
+
+    expect(findByBorderColor(Colors.red), findsOneWidget);
+  });
+
+  testWidgets('FilterChip displays the correct border color when not selected', (WidgetTester tester) async {
+    await tester.pumpWidget(
+      const example.WidgetStateBorderSideExampleApp(),
+    );
+
+    await tester.tap(find.byType(FilterChip));
+    await tester.pumpAndSettle();
+
+    final ThemeData theme = Theme.of(tester.element(find.byType(FilterChip)));
+
+    // FilterChip's border color defaults to ColorScheme.outlineVariant.
+    expect(
+      findByBorderColor(theme.colorScheme.outlineVariant),
+      findsOneWidget,
+    );
+  });
+}

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

@@ -480,6 +480,13 @@ class _EnabledAndDisabledMouseCursor extends WidgetStateMouseCursor {
 /// property values. [WidgetStateBorderSide] should only be used with widgets that document
 /// their support, like [ActionChip.side].
 ///
+/// {@tool dartpad}
+/// This example defines a [WidgetStateBorderSide] which resolves to different
+/// border colors depending on how the user interacts with it.
+///
+/// ** See code in examples/api/lib/widgets/widget_state/widget_state_border_side.0.dart **
+/// {@end-tool}
+///
 /// This class should only be used for parameters which are documented to take
 /// [WidgetStateBorderSide], otherwise only the default state will be used.
 ///