Add test for selectable_region.0.dart (#157258)

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

It adds a test for
- `examples/api/test/material/selectable_region/selectable_region.0_test.dart`

dev/bots/check_code_samples.dart

@@ -309,7 +309,6 @@ class SampleChecker {
 // TODO(gspencergoog): implement the missing tests.
 // See https://github.com/flutter/flutter/issues/130459
 final Set<String> _knownMissingTests = <String>{
-  'examples/api/test/material/selectable_region/selectable_region.0_test.dart',
   'examples/api/test/material/color_scheme/dynamic_content_color.0_test.dart',
   'examples/api/test/material/platform_menu_bar/platform_menu_bar.0_test.dart',
   'examples/api/test/painting/star_border/star_border.0_test.dart',

examples/api/lib/material/selectable_region/selectable_region.0.dart

@@ -322,6 +322,8 @@ class _RenderSelectableAdapter extends RenderProxyBox with Selectable, Selection
   @override
   void dispose() {
     _geometry.dispose();
+    _startHandle = null;
+    _endHandle = null;
     super.dispose();
   }
 }

examples/api/test/material/selectable_region/selectable_region.0_test.dart

@@ -0,0 +1,53 @@
+// 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/foundation.dart';
+import 'package:flutter/material.dart';
+import 'package:flutter/services.dart';
+import 'package:flutter_api_samples/material/selectable_region/selectable_region.0.dart' as example;
+import 'package:flutter_test/flutter_test.dart';
+
+void main() {
+  Future<void> sendKeyCombination(
+    WidgetTester tester,
+    LogicalKeyboardKey key,
+  ) async {
+    final LogicalKeyboardKey modifier = switch (defaultTargetPlatform) {
+      TargetPlatform.iOS || TargetPlatform.macOS => LogicalKeyboardKey.meta,
+      _ => LogicalKeyboardKey.control,
+    };
+    await tester.sendKeyDownEvent(modifier);
+    await tester.sendKeyDownEvent(key);
+    await tester.sendKeyUpEvent(key);
+    await tester.sendKeyUpEvent(modifier);
+    await tester.pump();
+  }
+
+  testWidgets('The icon can be selected with the text', (WidgetTester tester) async {
+    String? clipboard;
+    tester.binding.defaultBinaryMessenger.setMockMethodCallHandler(SystemChannels.platform, (MethodCall methodCall) async {
+      if (methodCall.method == 'Clipboard.setData') {
+        clipboard = (methodCall.arguments as Map<String, dynamic>)['text'] as String;
+      }
+      return null;
+    });
+    addTearDown(() {
+      TestDefaultBinaryMessengerBinding.instance.defaultBinaryMessenger.setMockMethodCallHandler(SystemChannels.platform, null);
+    });
+
+    await tester.pumpWidget(
+      const example.SelectableRegionExampleApp(),
+    );
+
+    await tester.tap(find.byIcon(Icons.key)); // Focus the application.
+    await tester.pump();
+
+    // Keyboard select all.
+    await sendKeyCombination(tester,LogicalKeyboardKey.keyA);
+    // Keyboard copy.
+    await sendKeyCombination(tester,LogicalKeyboardKey.keyC);
+
+    expect(clipboard, 'SelectableRegion SampleSelect this iconCustom Text');
+  }, variant: TargetPlatformVariant.all());
+}