dartfmt: Support generalized void

[eernstg]

This is the dartfmt specific issue for dart-lang/sdk#30176, which has the details.

[zoechi]

seems to be a blocker now

[munificent]

I just tried this with both the version of dart_style in the Dart SDK (which means using the bleeding edge analyzer) and the external dart_style package using analyzer 0.31.1 and this was formatted fine:

main() {
  List<void> nothings;
}

I still need to add tests but as far as I can tell, it's working. Is there something I'm missing?

[leafpetersen]

Re-opening: the following fails to parse and format:

typedef G2 = void Function([void]);

[munificent]

Blocked on dart-lang/sdk#30177.

[natebosch]

This is working on the given example and the linked SDK issue is resolved.