Treat org-dartlang-app URIs as VM source like dart: libraries
Treat org-dartlang-app URIs as VM source like dart: libraries

This isn't perfect, since VS Code won't know this is the same file as one on disk, but it at least lets you see/step through the source.

Fixes #2784.

Dart-Code/Dart-Code,Dart-Code/Dart-Code,Dart-Code/Dart-Code,Dart-Code/Dart-Code,Dart-Code/Dart-Code