[Dot Shorthands] VS Code Support

[kallentu]

This issue tracks the work needed for VS Code support.

Dart Code issue filed: Dart-Code/Dart-Code#5470

cc. @jwren

[DanTup]

I suspect there won't be anything to do specifically for Dart-Code (since all language parsing is in the analyzer/server), although I'll keep this open to verify we don't need any changes to the textmate grammar since that does somewhat parse Dart (albeit in a very simplified way).

[kallentu]

That sounds good, thank you.

[DanTup]

I've confirmed that using the new syntax, there is no difference in colouring with the textmate grammar (which mostly leaves identifiers like these white, but invocations yellow):

Semantic Tokens might be different (which is what most users will see overlaid over the textmate grammar), but the implementation of them is in the server (and included in the checklist at #59836).