Errors when hovering over enum values in VSCode during debugging

[annagrin]

See the issue and repro in Dart-Code/Dart-Code#3082

According to the trace, it looks like dwds fails to find a library reference for dart:_runtime during getObject() call for id ("classes|dart:_runtime|_Type")

To solve this properly we need to communicate the list of implicitly imported libraries from dart_sdk to the debugger:

Part of: #1290

[annagrin]

The issue is caused by missing implicit imports: dart-lang/sdk#45477, needs app-level metadata to resolve properly.
Working on a workaround for now (creating the missing metadata manually in the debugger).