Add test for image.frame_builder.0.dart (flutter#158247)

Contributes to flutter#130459

It adds a test for
- `examples/api/lib/widgets/image/image.frame_builder.0.dart`

dev/bots/check_code_samples.dart

@@ -310,5 +310,4 @@ class SampleChecker {
 // See https://github.com/flutter/flutter/issues/130459
 final Set<String> _knownMissingTests = <String>{
   'examples/api/test/material/color_scheme/dynamic_content_color.0_test.dart',
-  'examples/api/test/widgets/image/image.frame_builder.0_test.dart',
 };

examples/api/test/widgets/image/image.frame_builder.0_test.dart

@@ -0,0 +1,58 @@
+// 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/widgets/image/image.frame_builder.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 frame builder returns an AnimatedOpacity when not synchronously loaded', (WidgetTester tester) async {
+    await tester.pumpWidget(
+      const example.FrameBuilderExampleApp(),
+    );
+    await tester.pumpAndSettle();
+
+    final Image image = tester.widget<Image>(find.byType(Image));
+    final ImageFrameBuilder frameBuilder = image.frameBuilder!;
+    final BuildContext context = tester.element(find.byType(Image));
+
+    const Key key = Key('child');
+
+    expect(
+      frameBuilder(context, const SizedBox(key: key), null,  false),
+      isA<AnimatedOpacity>().having(
+        (AnimatedOpacity opacity) => opacity.child!.key, 'key', key,
+      ),
+    );
+  });
+
+  testWidgets('The frame builder returns the child when synchronously loaded', (WidgetTester tester) async {
+    await tester.pumpWidget(
+      const example.FrameBuilderExampleApp(),
+    );
+    await tester.pumpAndSettle();
+
+    final Image image = tester.widget<Image>(find.byType(Image));
+    final ImageFrameBuilder frameBuilder = image.frameBuilder!;
+    final BuildContext context = tester.element(find.byType(Image));
+
+    const Key key = Key('child');
+
+    expect(
+      frameBuilder(context, const SizedBox(key: key), null,  true),
+      isA<SizedBox>().having(
+        (SizedBox widget) => widget.key, 'key', key,
+      ),
+    );
+  });
+}