Seen on Firebase Studio with v3.114.0:

workbench.js:631 Activating extension 'Dart-Code.dart-code' failed: Extension 'Dart-Code.dart-code' CANNOT use API proposal: lmTools.
Its package.json#enabledApiProposals-property declares: [] but NOT lmTools.
The missing proposal MUST be added and you must start in extension development mode or use the following command line switch: --enable-proposed-api Dart-Code.dart-code.