[ffigen] `ObjCBlock.listener` refcounting issues

[liamappelbe]

ObjCBlock.listener lets ObjC code invoke Dart callbacks that live in other isolates. If they pass an object arg, it's possible that it gets refcounted and destroyed before the Dart callback runs (the Dart code calls retain, but it may be too late). So while these listener callbacks let us get rid of a lot of NativePort boilerplate, they don't totally eliminate the need for native code.

These trampolines are super simple and can be automatically generated by ffigen. But automatically compiling them relies on native_assets.