Allow extension type to implement Record and Function types about language

[nate-thegrate]

Only allowing implementation of named types was a deliberate choice—you can see an explanation at #3693 (comment).

[nate-thegrate]

@mmcdon20 fair point!

I don't see this being implemented anytime soon, but I agree that it'd be really nice :)

[mmcdon20]

@nate-thegrate It sounds like it would be possible to allow this behavior, just that it hasn't been done yet due to being more complicated than named types to implement.

Also in some sense, the feature partially exists already in that you can cast the extension type to the representation type successfully.

extension type Coordinate(({double latitude, double longitude}) _) {}

void main() {
  Coordinate coordinate = Coordinate((latitude: 5, longitude: 10));

  ({double latitude, double longitude}) tuple =
      coordinate as ({double latitude, double longitude});

  print(tuple.latitude);
}

But it would be a lot more convenient to use if it did not require casting.