test focus example 0 (#147564)

Part of #130459. Adds tests for [Focus example 0](https://api.flutter.dev/flutter/widgets/Focus-class.html#widgets.Focus.1).

dev/bots/check_code_samples.dart

@@ -463,7 +463,6 @@ final Set<String> _knownMissingTests = <String>{
   'examples/api/test/widgets/actions/focusable_action_detector.0_test.dart',
   'examples/api/test/widgets/color_filter/color_filtered.0_test.dart',
   'examples/api/test/widgets/focus_scope/focus.2_test.dart',
-  '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/scroll_view/custom_scroll_view.1_test.dart',

examples/api/test/widgets/focus_scope/focus.0_test.dart

@@ -0,0 +1,80 @@
+// 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/services.dart';
+import 'package:flutter_api_samples/widgets/focus_scope/focus.0.dart'
+  as example;
+import 'package:flutter_test/flutter_test.dart';
+
+void main() {
+  testWidgets('shows initial content', (WidgetTester tester) async {
+    await tester.pumpWidget(const example.FocusExampleApp());
+    expect(find.byType(Scaffold), findsOneWidget);
+    expect(find.byType(AppBar), findsOneWidget);
+    expect(find.byType(FocusScope), findsAtLeastNWidgets(1));
+    expect(find.text('Press to focus'), findsOneWidget);
+    expect(find.text('Focus Sample'), findsOneWidget);
+    expect(find.byType(Container), findsOneWidget);
+
+    final Container container =
+        tester.widget<Container>(find.byType(Container));
+    expect(container.color, Colors.white);
+  });
+
+  testWidgets('switches to focus mode', (WidgetTester tester) async {
+    await tester.pumpWidget(const example.FocusExampleApp());
+    expect(find.text('Press to focus'), findsOneWidget);
+    expect(find.text("I'm in color! Press R,G,B!"), findsNothing);
+    await tester.tap(find.text('Press to focus'));
+    await tester.pumpAndSettle();
+    expect(find.text('Press to focus'), findsNothing);
+    expect(find.text("I'm in color! Press R,G,B!"), findsOneWidget);
+
+    expect(find.byType(Container), findsOneWidget);
+    final Container container =
+    tester.widget<Container>(find.byType(Container));
+    expect(container.color, Colors.white);
+
+    await tester.tap(find.text("I'm in color! Press R,G,B!"));
+    await tester.pumpAndSettle();
+    expect(find.text('Press to focus'), findsOneWidget);
+    expect(find.text("I'm in color! Press R,G,B!"), findsNothing);
+  });
+
+  testWidgets('changes color according to key presses', (WidgetTester tester) async {
+    await tester.pumpWidget(const example.FocusExampleApp());
+    expect(find.byType(Container), findsOneWidget);
+    expect(tester.widget<Container>(find.byType(Container)).color, Colors.white);
+    await tester.sendKeyEvent(LogicalKeyboardKey.keyR);
+    await tester.pumpAndSettle();
+    expect(tester.widget<Container>(find.byType(Container)).color, Colors.white);
+    await tester.sendKeyEvent(LogicalKeyboardKey.keyG);
+    await tester.pumpAndSettle();
+    expect(tester.widget<Container>(find.byType(Container)).color, Colors.white);
+    await tester.sendKeyEvent(LogicalKeyboardKey.keyB);
+    await tester.pumpAndSettle();
+    expect(tester.widget<Container>(find.byType(Container)).color, Colors.white);
+
+    expect(find.text('Press to focus'), findsOneWidget);
+    await tester.tap(find.text('Press to focus'));
+    await tester.pumpAndSettle();
+    expect(find.byType(Container), findsOneWidget);
+    expect(tester.widget<Container>(find.byType(Container)).color, Colors.white);
+    await tester.sendKeyEvent(LogicalKeyboardKey.keyR);
+    await tester.pumpAndSettle();
+    expect(tester.widget<Container>(find.byType(Container)).color, Colors.red);
+    await tester.sendKeyEvent(LogicalKeyboardKey.keyG);
+    await tester.pumpAndSettle();
+    expect(tester.widget<Container>(find.byType(Container)).color, Colors.green);
+    await tester.sendKeyEvent(LogicalKeyboardKey.keyB);
+    await tester.pumpAndSettle();
+    expect(tester.widget<Container>(find.byType(Container)).color, Colors.blue);
+
+    expect(find.text("I'm in color! Press R,G,B!"), findsOneWidget);
+    await tester.tap(find.text("I'm in color! Press R,G,B!"));
+    await tester.pumpAndSettle();
+    expect(tester.widget<Container>(find.byType(Container)).color, Colors.white);
+  });
+}