Add a WebSocket.upgrade(Stream<List<int>>, StreamSink<List<int>>) static method

[natebosch]

We currently have over a thousand lines of copy/pasted code from dart:io to have an implementation of WebSocket which does not reference any other types from dart:io like Socket.

If we remove the API from package:web_socket_channel then in the known use case we can likely import dart:io and use this proposed API instead.
dart-lang/web_socket_channel#150

I'm not sure if we'd also want it to cover some of the behavior implemented in package:shelf_web_socket around prepping the upgrade and searching for protocols - we likely do want that.

https://github.com/dart-lang/shelf_web_socket/blob/fe1ce13114bb7318e140587f21ebec4cbb127ef3/lib/src/web_socket_handler.dart#L71-L83

cc @lrhn for thoughts

[natebosch]

This could also help make it easier to do the upgrades from a normal socket. #34284