Also, Dart has code generation so you can write your own language features, like the freezed package which implements algebraic data types even though Dart doesn't have them natively.
I'm planning on writing a library in Dart that implements a custom TCP protocol, and I think ADTs would be the best fit for capturing all the valid states and error conditions. I really don't want to have a bunch of nullable fields or use the visitor pattern :/
I've been tracking this issue [0] and hopefully with the release of null safety, the team will have the time to implement ADTs!