Transition of platform channel test interfaces to flutter_test package

Summary

Context

Description of change

Migration guide

Timeline

References

Summary

Context

As part of a refactoring of the low-level plugin
communications architecture, we have moved from the
previous onPlatformMessage/handlePlatformMessage
logic to a per-channel buffering system implemented in
the engine in the ChannelBuffers class.
To maintain compatibility with existing code,
the existing BinaryMessenger.setMessageHandler API
has been refactored to use the new ChannelBuffers API.

One difference between the ChannelBuffers API and the
previous API is that the new API is more consistent in
its approach to asynchrony. As a side-effect,
the APIs around message passing are now entirely asynchronous.