Unify Web sockets event callback styles across dart:html and dart:io

[sethladd]

Web sockets in dart:html:

  ws.on.open.add((e) {
    print("connected"):
  });

  ws.on.message.add((e) {
    print('received message ${e.data}');
  });

Web sockets in dart:io:

webSocketHandler.onOpen = (connection) {
  connection.onOpen = (message) {
    print(message);
  };
}

This difference leads to developer confusion. Why the difference? Should I emulate one or the other?

Let's unify the way callbacks are registered and handled across web sockets for HTML and IO. This is a specialization of the broader effort to create more unified looking libraries.

[sethladd]

Added Duplicate label.
Marked as being merged into #2958.

[kevmoo]

Removed Area-IO, Area-DOM labels.
Added Area-Library, Library-IO labels.