Something I immediately noticed is that because Cubit inherits from Stream, it surfaces the entire Stream API. This seems like a leaky abstraction, and I'm wondering whether there is some way that the API could be hidden?

One option I could see is for Cubit to have a protected/private CubitStream<State> stream property, rather than inherit from it. The most minimal change might be something like the following.

abstract class Cubit<State> {
  @protected
  CubitStream<State> stream;

  State get state => _stream.state;
}