Add a ListOutputStream.onData callback.

runtime/bin/websocket_impl.dart

 // Copyright (c) 2012, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 final String _webSocketGUID = "258EAFA5-E914-47DA-95CA-C5AB0DC85B11";
 
 class _WebSocketMessageType {
   static final int NONE = 0;
   static final int BINARY = 1;
   static final int TEXT = 2;
@@ skipping 485 matching lines @@
     } else {
       _outputStream.write(buffer.getRange(offset, count));
     }
   }
 
   _onWebSocketMessageEnd() {
     if (_onMessage != null) {
       if (_currentMessageType == _WebSocketMessageType.TEXT) {
         _onMessage(_decoder.decoded);
       } else {
-        _onMessage(_outputStream.contents());
+        _onMessage(_outputStream.read());
       }
     }
     _decoder = null;
     _outputStream = null;
   }
 
   _onWebSocketPing(List<int> payload) {
     _sendFrame(_WebSocketOpcode.PONG, payload);
   }
 
@@ skipping 364 matching lines @@
 
 class _WebSocketCloseEvent implements CloseEvent {
   _WebSocketCloseEvent(this._wasClean, this._code, this._reason);
   bool get wasClean() => _wasClean;
   int get code() => _code;
   String get reason() => _reason;
   bool _wasClean;
   int _code;
   String _reason;
 }