Add test for flexible_space_bar.0.dart (#157107)

Contributes to https://github.com/flutter/flutter/issues/130459

It adds a test for
- `examples/api/test/material/flexible_space_bar/flexible_space_bar.0_test.dart`

dev/bots/check_code_samples.dart

@@ -312,7 +312,6 @@ final Set<String> _knownMissingTests = <String>{
   'examples/api/test/material/selectable_region/selectable_region.0_test.dart',
   'examples/api/test/material/color_scheme/dynamic_content_color.0_test.dart',
   'examples/api/test/material/platform_menu_bar/platform_menu_bar.0_test.dart',
-  'examples/api/test/material/flexible_space_bar/flexible_space_bar.0_test.dart',
   'examples/api/test/material/navigation_rail/navigation_rail.extended_animation.0_test.dart',
   'examples/api/test/painting/star_border/star_border.0_test.dart',
   'examples/api/test/widgets/navigator/navigator.restorable_push_and_remove_until.0_test.dart',

examples/api/lib/material/flexible_space_bar/flexible_space_bar.0.dart

@@ -2,18 +2,23 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
+import 'package:flutter/gestures.dart';
 import 'package:flutter/material.dart';
 
 /// Flutter code sample for [FlexibleSpaceBar].
 
-void main() => runApp(const MaterialApp(home: FlexibleSpaceBarExampleApp()));
+void main() => runApp(const FlexibleSpaceBarExampleApp());
 
 class FlexibleSpaceBarExampleApp extends StatelessWidget {
   const FlexibleSpaceBarExampleApp({super.key});
 
   @override
   Widget build(BuildContext context) {
-    return Scaffold(
+    return MaterialApp(
+      scrollBehavior: const MaterialScrollBehavior().copyWith(
+        dragDevices: PointerDeviceKind.values.toSet(),
+      ),
+      home: Scaffold(
         body: CustomScrollView(
           physics: const BouncingScrollPhysics(parent: AlwaysScrollableScrollPhysics()),
           slivers: <Widget>[
@@ -74,6 +79,7 @@ class FlexibleSpaceBarExampleApp extends StatelessWidget {
             ),
           ],
         ),
+      ),
     );
   }
 }

examples/api/test/material/flexible_space_bar/flexible_space_bar.0_test.dart

@@ -0,0 +1,42 @@
+// 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:io';
+
+import 'package:flutter/material.dart';
+import 'package:flutter_api_samples/material/flexible_space_bar/flexible_space_bar.0.dart' as example;
+import 'package:flutter_test/flutter_test.dart';
+
+void main() {
+  // The app being tested loads images via HTTP which the test
+  // framework defeats by default.
+  setUpAll(() {
+    HttpOverrides.global = null;
+  });
+
+  testWidgets('The app bar stretches when over-scrolled', (WidgetTester tester) async {
+    await tester.pumpWidget(
+        const example.FlexibleSpaceBarExampleApp(),
+    );
+
+    expect(find.text('Flight Report'), findsOne);
+
+    expect(find.widgetWithText(ListTile, 'Sunday'), findsOne);
+    expect(find.widgetWithText(ListTile, 'Monday'), findsOne);
+    expect(find.text('sunny, h: 80, l: 65'), findsExactly(2));
+    expect(find.byIcon(Icons.wb_sunny), findsExactly(2));
+
+    final Finder appBarContainer = find.byType(Image);
+    final Size sizeBeforeScroll = tester.getSize(appBarContainer);
+    final Offset target = tester.getCenter(find.byType(ListTile).first);
+    final TestGesture gesture = await tester.startGesture(target);
+    await gesture.moveBy(const Offset(0.0, 100.0));
+    await tester.pump(const Duration(milliseconds: 10));
+    await gesture.up();
+    final Size sizeAfterScroll = tester.getSize(appBarContainer);
+
+    expect(sizeBeforeScroll.height, lessThan(sizeAfterScroll.height));
+    // Verifies ScrollBehavior.dragDevices is correctly set.
+  }, variant: TargetPlatformVariant.all());
+}