* [flutter_tools] Forward app.webLaunchUrl event from Flutter to DAP clients

Fixes https://github.com/Dart-Code/Dart-Code/issues/4292.

* Update packages/flutter_tools/lib/src/debug_adapters/flutter_adapter.dart
Co-authored-by: Christopher Fujino <fujino@google.com>

* Another static const instead of final
Co-authored-by: Christopher Fujino <fujino@google.com>