Get connection state

Get connection state in order to know if the socket could connect to an url.
in HTMLWebSocketChannel you only need to expose _webSocket.readyState attribute as:

int get readyState => _webSocket.readyState;

Start at HTMLWebSocketChannel and inspect how the underlying _webSocket is exposed. Verify the public readyState value mirrors the socket's readyState, then run the existing test suite or relevant channel tests if available; done means callers can read the connection state.

Written by the indexing model from the issue text.