Add test for future_builder.0.dart (#148453)

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

It adds a test for
- `examples/api/lib/widgets/async/future_builder.0.dart`

dev/bots/check_code_samples.dart

@@ -378,7 +378,6 @@ final Set<String> _knownMissingTests = <String>{
   'examples/api/test/widgets/nested_scroll_view/nested_scroll_view_state.0_test.dart',
   'examples/api/test/widgets/scroll_position/scroll_metrics_notification.0_test.dart',
   'examples/api/test/widgets/media_query/media_query_data.system_gesture_insets.0_test.dart',
-  'examples/api/test/widgets/async/future_builder.0_test.dart',
   'examples/api/test/widgets/animated_switcher/animated_switcher.0_test.dart',
   'examples/api/test/widgets/animated_list/animated_list.0_test.dart',
   'examples/api/test/widgets/focus_traversal/focus_traversal_group.0_test.dart',

examples/api/test/widgets/async/future_builder.0_test.dart

@@ -0,0 +1,29 @@
+// 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';
+import 'package:flutter_api_samples/widgets/async/future_builder.0.dart'
+    as example;
+import 'package:flutter_test/flutter_test.dart';
+
+void main() {
+  testWidgets('StreamBuilder listens to internal stream', (WidgetTester tester) async {
+    await tester.pumpWidget(
+      const example.FutureBuilderExampleApp(),
+    );
+
+    expect(find.byType(CircularProgressIndicator), findsOne);
+    expect(find.text('Awaiting result...'), findsOne);
+
+    await tester.pump(const Duration(seconds: 1));
+
+    expect(find.byType(CircularProgressIndicator), findsOne);
+    expect(find.text('Awaiting result...'), findsOne);
+
+    await tester.pump(const Duration(seconds: 1));
+
+    expect(find.byIcon(Icons.check_circle_outline), findsOne);
+    expect(find.text('Result: Data Loaded'), findsOne);
+  });
+}