Add test for icon_button.3.dart (#149988)

Contributes to flutter/flutter#130459

It adds a test for
- `examples/api/lib/material/icon_button/icon_button.3.dart`

dev/bots/check_code_samples.dart

@@ -324,7 +324,6 @@ final Set<String> _knownMissingTests = <String>{
   'examples/api/test/material/stepper/stepper.controls_builder.0_test.dart',
   'examples/api/test/material/flexible_space_bar/flexible_space_bar.0_test.dart',
   'examples/api/test/material/chip/deletable_chip_attributes.on_deleted.0_test.dart',
-  'examples/api/test/material/icon_button/icon_button.3_test.dart',
   'examples/api/test/material/expansion_panel/expansion_panel_list.expansion_panel_list_radio.0_test.dart',
   'examples/api/test/material/input_decorator/input_decoration.1_test.dart',
   'examples/api/test/material/input_decorator/input_decoration.prefix_icon_constraints.0_test.dart',

examples/api/test/material/icon_button/icon_button.3_test.dart

@@ -0,0 +1,50 @@
+// 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/material/icon_button/icon_button.3.dart' as example;
+import 'package:flutter_test/flutter_test.dart';
+
+void main() {
+  testWidgets('It should select and unselect the icon buttons', (WidgetTester tester) async {
+    await tester.pumpWidget(
+      const example.IconButtonToggleApp(),
+    );
+
+    expect(find.widgetWithIcon(IconButton, Icons.settings_outlined), findsExactly(8));
+    final Finder unselectedIconButtons = find.widgetWithIcon(IconButton, Icons.settings_outlined);
+    for (int i = 0; i <= 6; i++) {
+      final IconButton button = tester.widget<IconButton>(unselectedIconButtons.at(i));
+      expect(button.onPressed, i.isEven ? isA<VoidCallback>() : isNull);
+      expect(button.isSelected, isFalse);
+    }
+
+    // Select the icons buttons.
+    for (int i = 0; i <= 3; i++) {
+      await tester.tap(unselectedIconButtons.at(2 * i));
+    }
+    await tester.pump();
+
+    expect(find.widgetWithIcon(IconButton, Icons.settings), findsExactly(8));
+    final Finder selectedIconButtons = find.widgetWithIcon(IconButton, Icons.settings);
+    for (int i = 0; i <= 6; i++) {
+      final IconButton button = tester.widget<IconButton>(selectedIconButtons.at(i));
+      expect(button.onPressed, i.isEven ? isA<VoidCallback>() : isNull);
+      expect(button.isSelected, isTrue);
+    }
+
+    // Unselect the icons buttons.
+    for (int i = 0; i <= 3; i++) {
+      await tester.tap(selectedIconButtons.at(2 * i));
+    }
+    await tester.pump();
+
+    expect(find.widgetWithIcon(IconButton, Icons.settings_outlined), findsExactly(8));
+    for (int i = 0; i <= 6; i++) {
+      final IconButton button = tester.widget<IconButton>(unselectedIconButtons.at(i));
+      expect(button.onPressed, i.isEven ? isA<VoidCallback>() : isNull);
+      expect(button.isSelected, isFalse);
+    }
+  });
+}