Describe the bug
https://flutter.dev/docs/development/accessibility-and-localization/internationalization

In Flutter 1.22, a new tool for generating translations was released. This tool generates a synthetic package in the root
.dart_tool/flutter_gen/l10n_gen and updates the code when flutter run is running and the arb template file changes .

But the new code generated inside that package is not reanalyzed and saw an error like if the methods inside the package don't exists. Once the file .dart_tool/flutter_gen/l10n_gen/app_localizations.dart is opened the error disapears.

To Reproduce
Steps to reproduce the behavior:

1. Follow the steps at https://flutter.dev/docs/development/accessibility-and-localization/internationalization

Screenshots
If applicable, add screenshots to help explain your problem.

Versions (please complete the following information):

• VS Code version: 1.52.1
• Dart extension version: 3.18.1
• Dart/Flutter SDK version: Channel dev, 1.26.0-8.0.pre, on macOS 11.1 20C69 darwin-x64, locale es-E