Copy the web socket stuff from http_parser.

I'm moving this to a separate package where it makes sense to also have
implementations of WebSocketChannel that wrap the dart:io and dart:html
WebSocket classes. This will make it a lot easier for users to use
libraries (such as json_rpc_2) that require StreamChannels with
well-behaved socket channels.

R=kevmoo@google.com

Review URL: https://codereview.chromium.org//1734773002 .