When using a HostApi that stores an object and the object also uses a FlutterApi, it creates a circular reference:

Object--S-->FlutterApi--S-->BinaryMessenger--S-->HostApi--S-->Dictionary<int, Object>
^-----------------------------------------------S------------------------------------------------/

I would expect that if the engine would release their reference to the BinaryMessenger, then the host apis and flutter apis should no longer need to handle/invoke methods. Therefore, their reference to it could be weak.