Remove code copied from the SDK

[natebosch]

We currently have over a thousand lines of code to support a single API:

https://github.com/dart-lang/web_socket_channel/blob/c57baea6168b446a27745cf188fb0f54d539de2c/lib/src/channel.dart#L98-L102

That API has very little usage. I think the only Dart team usage may be from shelf_web_socket.

As far as I can tell, unless we either give access to the underlying Socket from the package:shelf APIs it is not possible to implement the shelf_web_socket behavior without this copied code from the SDK living somewhere.

We could consider adding a WebSocket.upgrade(Stream<List<int>>, StreamSink<List<int>>) static method in dart:io which I think would give us enough to remove the copied code.