Also thanks for adding sum types, and pattern matching, is there anyway you could get the dart team on board? They seem to be averse to them and it's a big reason why I don't use the Dart/Flutter ecosystem.
I'm driving the design of pattern matching for Dart. The entire language team is on board with them and we have a design that's pretty far along. It's really hard to integrate pattern matching into a language that wasn't originally designed for it, so it's taken a while to settle on a syntax that makes sense, but I think we're getting close.
If you'd like to be involved, there is a category on the issue tracker for design discussions related to patterns and records:
https://github.com/dart-lang/language/issues?q=is%3Aissue+is...