Errors when hovering over enum values in VSCode during debugging

See the issue and repro in https://github.com/Dart-Code/Dart-Code/issues/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: https://github.com/dart-lang/webdev/issues/1290

Start with the linked Dart-Code issue and reproduction, then inspect the dwds trace for the getObject() call using id "classes|dart:_runtime|_Type". Trace how dart_sdk's implicitly imported libraries are represented and communicated to the debugger; done means enum values no longer produce hover errors in the linked VSCode debugging reproduction.

Written by the indexing model from the issue text.