Add test for dropdown_menu.1.dart (#149547)

Contributes to flutter/flutter#130459

It adds a test for
- `examples/api/lib/material/dropdown_menu/dropdown_menu.1.dart`

dev/bots/check_code_samples.dart

@@ -337,7 +337,6 @@ final Set<String> _knownMissingTests = <String>{
   'examples/api/test/material/input_decorator/input_decoration.material_state.1_test.dart',
   'examples/api/test/material/text_form_field/text_form_field.1_test.dart',
   'examples/api/test/material/scrollbar/scrollbar.1_test.dart',
-  'examples/api/test/material/dropdown_menu/dropdown_menu.1_test.dart',
   'examples/api/test/material/search_anchor/search_anchor.0_test.dart',
   'examples/api/test/material/search_anchor/search_anchor.1_test.dart',
   'examples/api/test/material/search_anchor/search_anchor.2_test.dart',

examples/api/test/material/dropdown_menu/dropdown_menu.1_test.dart

@@ -0,0 +1,56 @@
+// 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/dropdown_menu/dropdown_menu.1.dart' as example;
+import 'package:flutter_test/flutter_test.dart';
+
+void main() {
+  testWidgets('The DropdownMenu should display a menu with the list of entries the user can select', (WidgetTester tester) async {
+    await tester.pumpWidget(
+      const example.DropdownMenuApp(),
+    );
+
+    expect(find.widgetWithText(TextField, 'One'), findsOne);
+    final Finder menu = find.byType(DropdownMenu<String>);
+    expect(menu, findsOne);
+
+    Finder findMenuItem(String label) {
+      return find.widgetWithText(MenuItemButton, label).last;
+    }
+
+    await tester.tap(menu);
+    await tester.pumpAndSettle();
+    expect(findMenuItem('One'), findsOne);
+    expect(findMenuItem('Two'), findsOne);
+    expect(findMenuItem('Three'), findsOne);
+    expect(findMenuItem('Four'), findsOne);
+
+    await tester.tap(findMenuItem('Two'));
+
+    // The DropdownMenu's onSelected callback is delayed
+    // with SchedulerBinding.instance.addPostFrameCallback
+    // to give the focus a chance to return to where it was
+    // before the menu appeared. The pumpAndSettle()
+    // give the callback a chance to run.
+    await tester.pumpAndSettle();
+
+    expect(find.widgetWithText(TextField, 'Two'), findsOne);
+  });
+
+  testWidgets('DropdownMenu has focus when tapping on the text field', (WidgetTester tester) async {
+    await tester.pumpWidget(
+      const example.DropdownMenuApp(),
+    );
+
+    // Make sure the dropdown menus are there.
+    final Finder menu = find.byType(DropdownMenu<String>);
+    expect(menu, findsOne);
+
+    // Tap on the menu and make sure it is focused.
+    await tester.tap(menu);
+    await tester.pumpAndSettle();
+    expect(FocusScope.of(tester.element(menu)).hasFocus, isTrue);
+  });
+}