### Description
- Fixes https://github.com/flutter/flutter/issues/134205.

### Tests
- Removes ignoring the `_RestorableRouteInformation` leak from `cupertino/app_test.dart`;
- Removes ignoring the `_RestorableRouteInformation` leak from `material/app_test.dart`;
- Removes ignoring the `_RestorableRouteInformation` leak from `widgets/app_test.dart`;
- Removes ignoring the `_RestorableRouteInformation` leak from `widgets/route_notification_messages_test.dart`;
- Removes ignoring the `_RestorableRouteInformation` leak from `widgets/router_restoration_test.dart`;
- Updates `widgets/router_test.dart` to use `testWidgetsWithLeakTracking`.