Add tests for single_activator.0.dart API example. (#147426)

This PR contributes to https://github.com/flutter/flutter/issues/130459

### Description
- Fixes name of the `examples/api/lib/widgets/shortcuts/single_activator.single_activator.0.dart`
- Adds tests for `examples/api/lib/widgets/shortcuts/single_activator.0.dart`

dev/bots/check_code_samples.dart

@@ -433,7 +433,6 @@ final Set<String> _knownMissingTests = <String>{
   'examples/api/test/widgets/image/image.loading_builder.0_test.dart',
   'examples/api/test/widgets/shortcuts/logical_key_set.0_test.dart',
   'examples/api/test/widgets/shortcuts/shortcuts.0_test.dart',
-  'examples/api/test/widgets/shortcuts/single_activator.single_activator.0_test.dart',
   'examples/api/test/widgets/shortcuts/shortcuts.1_test.dart',
   'examples/api/test/widgets/shortcuts/callback_shortcuts.0_test.dart',
   'examples/api/test/widgets/page_storage/page_storage.0_test.dart',

examples/api/lib/widgets/shortcuts/single_activator.0.dart

@@ -5,7 +5,7 @@
 import 'package:flutter/material.dart';
 import 'package:flutter/services.dart';
 
-/// Flutter code sample for [SingleActivator.SingleActivator].
+/// Flutter code sample for [SingleActivator].
 
 void main() => runApp(const SingleActivatorExampleApp());
 

examples/api/test/widgets/shortcuts/single_activator.0_test.dart

@@ -0,0 +1,47 @@
+// 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/services.dart';
+import 'package:flutter_api_samples/widgets/shortcuts/single_activator.0.dart'
+    as example;
+import 'package:flutter_test/flutter_test.dart';
+
+void main() {
+  Future<void> pressControlC(WidgetTester tester) async {
+    await tester.sendKeyDownEvent(LogicalKeyboardKey.control);
+    await tester.sendKeyDownEvent(LogicalKeyboardKey.keyC);
+    await tester.sendKeyUpEvent(LogicalKeyboardKey.keyC);
+    await tester.sendKeyUpEvent(LogicalKeyboardKey.control);
+  }
+
+  group('SingleActivatorExampleApp', () {
+    testWidgets('displays correct labels', (WidgetTester tester) async {
+      await tester.pumpWidget(
+        const example.SingleActivatorExampleApp(),
+      );
+
+      expect(
+        find.text('Add to the counter by pressing Ctrl+C'),
+        findsOneWidget,
+      );
+      expect(find.text('count: 0'), findsOneWidget);
+    });
+
+    testWidgets(
+      'updates counter when Ctrl-C combination pressed',
+      (WidgetTester tester) async {
+        await tester.pumpWidget(
+          const example.SingleActivatorExampleApp(),
+        );
+
+        for (int counter = 0; counter < 10; counter++) {
+          expect(find.text('count: $counter'), findsOneWidget);
+
+          await pressControlC(tester);
+          await tester.pump();
+        }
+      },
+    );
+  });
+}

packages/flutter/lib/src/widgets/shortcuts.dart

@@ -432,7 +432,7 @@ class SingleActivator with Diagnosticable, MenuSerializableShortcut implements S
   /// In the following example, the shortcut `Control + C` increases the
   /// counter:
   ///
-  /// ** See code in examples/api/lib/widgets/shortcuts/single_activator.single_activator.0.dart **
+  /// ** See code in examples/api/lib/widgets/shortcuts/single_activator.0.dart **
   /// {@end-tool}
   const SingleActivator(
     this.trigger, {