Add tests for tween_animation_builder.0.dart API example. (#148902)

This PR contributes to https://github.com/flutter/flutter/issues/130459

### Description
- Adds tests for `examples/api/lib/widgets/tween_animation_builder/tween_animation_builder.0.dart`

dev/bots/check_code_samples.dart

@@ -393,7 +393,6 @@ final Set<String> _knownMissingTests = <String>{
   'examples/api/test/widgets/notification_listener/notification.0_test.dart',
   'examples/api/test/widgets/overscroll_indicator/glowing_overscroll_indicator.1_test.dart',
   'examples/api/test/widgets/overscroll_indicator/glowing_overscroll_indicator.0_test.dart',
-  'examples/api/test/widgets/tween_animation_builder/tween_animation_builder.0_test.dart',
   'examples/api/test/widgets/single_child_scroll_view/single_child_scroll_view.1_test.dart',
   'examples/api/test/widgets/single_child_scroll_view/single_child_scroll_view.0_test.dart',
   'examples/api/test/widgets/restoration/restoration_mixin.0_test.dart',

examples/api/lib/widgets/tween_animation_builder/tween_animation_builder.0.dart

@@ -15,7 +15,9 @@ class TweenAnimationBuilderExampleApp extends StatelessWidget {
   Widget build(BuildContext context) {
     return MaterialApp(
       home: Scaffold(
-        appBar: AppBar(title: const Text('TweenAnimationBuilder Sample')),
+        appBar: AppBar(
+          title: const Text('TweenAnimationBuilder Sample'),
+        ),
         body: const Center(
           child: TweenAnimationBuilderExample(),
         ),
@@ -28,16 +30,18 @@ class TweenAnimationBuilderExample extends StatefulWidget {
   const TweenAnimationBuilderExample({super.key});
 
   @override
-  State<TweenAnimationBuilderExample> createState() => _TweenAnimationBuilderExampleState();
+  State<TweenAnimationBuilderExample> createState() =>
+      _TweenAnimationBuilderExampleState();
 }
 
-class _TweenAnimationBuilderExampleState extends State<TweenAnimationBuilderExample> {
-  double targetValue = 24.0;
+class _TweenAnimationBuilderExampleState
+    extends State<TweenAnimationBuilderExample> {
+  double _targetValue = 24.0;
 
   @override
   Widget build(BuildContext context) {
     return TweenAnimationBuilder<double>(
-      tween: Tween<double>(begin: 0, end: targetValue),
+      tween: Tween<double>(begin: 0, end: _targetValue),
       duration: const Duration(seconds: 1),
       builder: (BuildContext context, double size, Widget? child) {
         return IconButton(
@@ -46,7 +50,7 @@ class _TweenAnimationBuilderExampleState extends State<TweenAnimationBuilderExam
           icon: child!,
           onPressed: () {
             setState(() {
-              targetValue = targetValue == 24.0 ? 48.0 : 24.0;
+              _targetValue = _targetValue == 24.0 ? 48.0 : 24.0;
             });
           },
         );

examples/api/test/widgets/tween_animation_builder/tween_animation_builder.0_test.dart

@@ -0,0 +1,160 @@
+// 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/tween_animation_builder/tween_animation_builder.0.dart'
+    as example;
+import 'package:flutter_test/flutter_test.dart';
+
+void main() {
+  testWidgets('Animates icon size on first build', (WidgetTester tester) async {
+    await tester.pumpWidget(
+      const example.TweenAnimationBuilderExampleApp(),
+    );
+
+    // The animation duration defined in the example app.
+    const Duration animationDuration = Duration(seconds: 1);
+
+    const double beginSize = 0.0;
+    const double endSize = 24.0;
+
+    final Finder iconButtonFinder = find.byType(IconButton);
+
+    IconButton iconButton = tester.widget(iconButtonFinder);
+    expect(iconButton.iconSize, equals(beginSize));
+
+    // Advance animation to the middle.
+    await tester.pump(animationDuration ~/ 2);
+
+    iconButton = tester.widget(iconButtonFinder);
+    expect(
+      iconButton.iconSize,
+      equals(lerpDouble(beginSize, endSize, 0.5)),
+    );
+
+    // Advance animation to the end.
+    await tester.pump(animationDuration ~/ 2);
+
+    iconButton = tester.widget(iconButtonFinder);
+    expect(iconButton.iconSize, equals(endSize));
+  });
+
+  testWidgets('Animates icon size on IconButton tap', (WidgetTester tester) async {
+    await tester.pumpWidget(
+      const example.TweenAnimationBuilderExampleApp(),
+    );
+    await tester.pumpAndSettle();
+
+    // The animation duration defined in the example app.
+    const Duration animationDuration = Duration(seconds: 1);
+
+    const double beginSize = 24.0;
+    const double endSize = 48.0;
+
+    final Finder iconButtonFinder = find.byType(IconButton);
+
+    IconButton iconButton = tester.widget(iconButtonFinder);
+    expect(iconButton.iconSize, equals(beginSize));
+
+    // Tap on the IconButton to start the forward animation.
+    await tester.tap(iconButtonFinder);
+    await tester.pump();
+
+    iconButton = tester.widget(iconButtonFinder);
+    expect(iconButton.iconSize, equals(beginSize));
+
+    // Advance animation to the middle.
+    await tester.pump(animationDuration ~/ 2);
+
+    iconButton = tester.widget(iconButtonFinder);
+    expect(
+      iconButton.iconSize,
+      equals(lerpDouble(beginSize, endSize, 0.5)),
+    );
+
+    // Advance animation to the end.
+    await tester.pump(animationDuration ~/ 2);
+
+    iconButton = tester.widget(iconButtonFinder);
+    expect(iconButton.iconSize, equals(endSize));
+
+    // Tap on the IconButton to start the reverse animation.
+    await tester.tap(iconButtonFinder);
+    await tester.pump();
+
+    iconButton = tester.widget(iconButtonFinder);
+    expect(iconButton.iconSize, equals(endSize));
+
+    // Advance animation to the middle.
+    await tester.pump(animationDuration ~/ 2);
+
+    iconButton = tester.widget(iconButtonFinder);
+    expect(
+      iconButton.iconSize,
+      equals(lerpDouble(endSize, beginSize, 0.5)),
+    );
+
+    // Advance animation to the end.
+    await tester.pump(animationDuration ~/ 2);
+
+    iconButton = tester.widget(iconButtonFinder);
+    expect(iconButton.iconSize, equals(beginSize));
+  });
+
+  testWidgets('Animation target can be updated during the animation', (WidgetTester tester) async {
+    await tester.pumpWidget(
+      const example.TweenAnimationBuilderExampleApp(),
+    );
+    await tester.pumpAndSettle();
+
+    // The animation duration defined in the example app.
+    const Duration animationDuration = Duration(seconds: 1);
+
+    const double beginSize = 24.0;
+    const double endSize = 48.0;
+    final double middleSize = lerpDouble(beginSize, endSize, 0.5)!;
+
+    final Finder iconButtonFinder = find.byType(IconButton);
+
+    IconButton iconButton = tester.widget(iconButtonFinder);
+    expect(iconButton.iconSize, equals(beginSize));
+
+    // Tap on the IconButton to start the forward animation.
+    await tester.tap(iconButtonFinder);
+    await tester.pump();
+
+    iconButton = tester.widget(iconButtonFinder);
+    expect(iconButton.iconSize, equals(beginSize));
+
+    // Advance animation to the middle.
+    await tester.pump(animationDuration ~/ 2);
+
+    iconButton = tester.widget(iconButtonFinder);
+    expect(iconButton.iconSize, equals(middleSize));
+
+    // Tap on the IconButton to start the backward animation.
+    await tester.tap(iconButtonFinder);
+    await tester.pump();
+
+    iconButton = tester.widget(iconButtonFinder);
+    expect(iconButton.iconSize, equals(middleSize));
+
+    // Advance animation to the middle.
+    await tester.pump(animationDuration ~/ 2);
+
+    iconButton = tester.widget(iconButtonFinder);
+    expect(
+      iconButton.iconSize,
+      equals(lerpDouble(middleSize, beginSize, 0.5)),
+    );
+
+    // Advance animation to the end.
+    await tester.pump(animationDuration ~/ 2);
+
+    iconButton = tester.widget(iconButtonFinder);
+    expect(iconButton.iconSize, equals(beginSize));
+  });
+}