Add test for animated_fractionally_sized_box.0.dart API example. (#146721)

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

### Description
- Adds `examples/api/test/widgets/implicit_animations/animated_fractionally_sized_box.0_test.dart` test

dev/bots/check_code_samples.dart

@@ -466,7 +466,6 @@ final Set<String> _knownMissingTests = <String>{
   'examples/api/test/widgets/focus_scope/focus.0_test.dart',
   'examples/api/test/widgets/focus_scope/focus.1_test.dart',
   'examples/api/test/widgets/focus_scope/focus_scope.0_test.dart',
-  'examples/api/test/widgets/implicit_animations/animated_fractionally_sized_box.0_test.dart',
   'examples/api/test/widgets/scroll_view/custom_scroll_view.1_test.dart',
   'examples/api/test/widgets/inherited_notifier/inherited_notifier.0_test.dart',
   'examples/api/test/animation/curves/curve2_d.0_test.dart',

examples/api/lib/widgets/implicit_animations/animated_fractionally_sized_box.0.dart

@@ -11,25 +11,43 @@ void main() => runApp(const AnimatedFractionallySizedBoxExampleApp());
 class AnimatedFractionallySizedBoxExampleApp extends StatelessWidget {
   const AnimatedFractionallySizedBoxExampleApp({super.key});
 
+  static const Duration duration = Duration(seconds: 1);
+  static const Curve curve = Curves.fastOutSlowIn;
+
   @override
   Widget build(BuildContext context) {
     return MaterialApp(
       home: Scaffold(
-        appBar: AppBar(title: const Text('AnimatedFractionallySizedBox Sample')),
-        body: const AnimatedFractionallySizedBoxExample(),
+        appBar: AppBar(
+          title: const Text('AnimatedFractionallySizedBox Sample'),
+        ),
+        body: const AnimatedFractionallySizedBoxExample(
+          duration: duration,
+          curve: curve,
+        ),
       ),
     );
   }
 }
 
 class AnimatedFractionallySizedBoxExample extends StatefulWidget {
-  const AnimatedFractionallySizedBoxExample({super.key});
+  const AnimatedFractionallySizedBoxExample({
+    required this.duration,
+    required this.curve,
+    super.key,
+  });
+
+  final Duration duration;
+
+  final Curve curve;
 
   @override
-  State<AnimatedFractionallySizedBoxExample> createState() => _AnimatedFractionallySizedBoxExampleState();
+  State<AnimatedFractionallySizedBoxExample> createState() =>
+      _AnimatedFractionallySizedBoxExampleState();
 }
 
-class _AnimatedFractionallySizedBoxExampleState extends State<AnimatedFractionallySizedBoxExample> {
+class _AnimatedFractionallySizedBoxExampleState
+    extends State<AnimatedFractionallySizedBoxExample> {
   bool selected = false;
 
   @override
@@ -50,8 +68,8 @@ class _AnimatedFractionallySizedBoxExampleState extends State<AnimatedFractional
               widthFactor: selected ? 0.25 : 0.75,
               heightFactor: selected ? 0.75 : 0.25,
               alignment: selected ? Alignment.topLeft : Alignment.bottomRight,
-              duration: const Duration(seconds: 1),
-              curve: Curves.fastOutSlowIn,
+              duration: widget.duration,
+              curve: widget.curve,
               child: const ColoredBox(
                 color: Colors.blue,
                 child: FlutterLogo(size: 75),

examples/api/test/widgets/implicit_animations/animated_fractionally_sized_box.0_test.dart

@@ -0,0 +1,125 @@
+// 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/implicit_animations/animated_fractionally_sized_box.0.dart'
+    as example;
+import 'package:flutter_test/flutter_test.dart';
+
+void main() {
+  testWidgets(
+    'AnimatedFractionallySizedBox animates on tap',
+    (WidgetTester tester) async {
+      await tester.pumpWidget(
+        const example.AnimatedFractionallySizedBoxExampleApp(),
+      );
+
+      final Finder fractionallySizedBoxFinder = find.descendant(
+        of: find.byType(AnimatedFractionallySizedBox),
+        matching: find.byType(FractionallySizedBox),
+      );
+
+      const double beginWidthFactor = 0.75;
+      const double endWidthFactor = 0.25;
+      const double beginHeightFactor = 0.25;
+      const double endHeightFactor = 0.75;
+      const Alignment beginAlignment = Alignment.bottomRight;
+      const Alignment endAlignment = Alignment.topLeft;
+
+      FractionallySizedBox fractionallySizedBox = tester.widget(
+        fractionallySizedBoxFinder,
+      );
+      expect(fractionallySizedBox.widthFactor, beginWidthFactor);
+      expect(fractionallySizedBox.heightFactor, beginHeightFactor);
+      expect(fractionallySizedBox.alignment, beginAlignment);
+
+      // Tap on the AnimatedFractionallySizedBoxExample to start the forward
+      // animation.
+      await tester.tap(
+        find.byType(example.AnimatedFractionallySizedBoxExample),
+      );
+      await tester.pump();
+
+      fractionallySizedBox = tester.widget(fractionallySizedBoxFinder);
+      expect(fractionallySizedBox.widthFactor, beginWidthFactor);
+      expect(fractionallySizedBox.heightFactor, beginHeightFactor);
+      expect(fractionallySizedBox.alignment, beginAlignment);
+
+      // Advance animation to the middle.
+      await tester.pump(
+        example.AnimatedFractionallySizedBoxExampleApp.duration ~/ 2,
+      );
+
+      final double t =
+          example.AnimatedFractionallySizedBoxExampleApp.curve.transform(0.5);
+
+      fractionallySizedBox = tester.widget(fractionallySizedBoxFinder);
+      expect(
+        fractionallySizedBox.widthFactor,
+        lerpDouble(beginWidthFactor, endWidthFactor, t),
+      );
+      expect(
+        fractionallySizedBox.heightFactor,
+        lerpDouble(beginHeightFactor, endHeightFactor, t),
+      );
+      expect(
+        fractionallySizedBox.alignment,
+        Alignment.lerp(beginAlignment, endAlignment, t),
+      );
+
+      // Advance animation to the end.
+      await tester.pump(
+        example.AnimatedFractionallySizedBoxExampleApp.duration ~/ 2,
+      );
+
+      fractionallySizedBox = tester.widget(fractionallySizedBoxFinder);
+      expect(fractionallySizedBox.widthFactor, endWidthFactor);
+      expect(fractionallySizedBox.heightFactor, endHeightFactor);
+      expect(fractionallySizedBox.alignment, endAlignment);
+
+      // Tap on the AnimatedFractionallySizedBoxExample again to start the
+      // reverse animation.
+      await tester.tap(
+        find.byType(example.AnimatedFractionallySizedBoxExample),
+      );
+      await tester.pump();
+
+      fractionallySizedBox = tester.widget(fractionallySizedBoxFinder);
+      expect(fractionallySizedBox.widthFactor, endWidthFactor);
+      expect(fractionallySizedBox.heightFactor, endHeightFactor);
+      expect(fractionallySizedBox.alignment, endAlignment);
+
+      // Advance animation to the middle.
+      await tester.pump(
+        example.AnimatedFractionallySizedBoxExampleApp.duration ~/ 2,
+      );
+
+      fractionallySizedBox = tester.widget(fractionallySizedBoxFinder);
+      expect(
+        fractionallySizedBox.widthFactor,
+        lerpDouble(endWidthFactor, beginWidthFactor, t),
+      );
+      expect(
+        fractionallySizedBox.heightFactor,
+        lerpDouble(endHeightFactor, beginHeightFactor, t),
+      );
+      expect(
+        fractionallySizedBox.alignment,
+        Alignment.lerp(endAlignment, beginAlignment, t),
+      );
+
+      // Advance animation to the end.
+      await tester.pump(
+        example.AnimatedFractionallySizedBoxExampleApp.duration ~/ 2,
+      );
+
+      fractionallySizedBox = tester.widget(fractionallySizedBoxFinder);
+      expect(fractionallySizedBox.widthFactor, beginWidthFactor);
+      expect(fractionallySizedBox.heightFactor, beginHeightFactor);
+      expect(fractionallySizedBox.alignment, beginAlignment);
+    },
+  );
+}