Fix a WebSocket crash.

sdk/lib/io/websocket_impl.dart

 // Copyright (c) 2013, 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.
 
 part of dart.io;
 
 const String _webSocketGUID = "258EAFA5-E914-47DA-95CA-C5AB0DC85B11";
 
 // Matches _WebSocketOpcode.
 class _WebSocketMessageType {
@@ skipping 885 matching lines @@
           }
           // Protocol close, use close code from transformer.
           _closeCode = transformer.closeCode;
           _closeReason = transformer.closeReason;
           _controller.close();
         },
         cancelOnError: true);
     _subscription.pause();
     _controller = new StreamController(sync: true,
                                        onListen: _subscription.resume,
+                                       onCancel: () {
+                                         _subscription.cancel();
+                                         _subscription = null;
+                                       },
                                        onPause: _subscription.pause,
                                        onResume: _subscription.resume);
 
     _webSockets[_serviceId] = this;
     try { _socket._owner = this; } catch (_) {}
   }
 
   StreamSubscription listen(void onData(message),
                             {Function onError,
                              void onDone(),
@@ skipping 42 matching lines @@
     if (_outCloseCode == null) {
       _outCloseCode = code;
       _outCloseReason = reason;
     }
     if (!_controller.isClosed) {
       // If a close has not yet been received from the other end then
       //   1) make sure to listen on the stream so the close frame will be
       //      processed if received.
       //   2) set a timer terminate the connection if a close frame is
       //      not received.
-      if (!_controller.hasListener) {
+      if (!_controller.hasListener && _subscription != null) {
         _controller.stream.drain().catchError((_) => {});
       }
       if (_closeTimer == null) {
         // When closing the web-socket, we no longer accept data.
         _closeTimer = new Timer(const Duration(seconds: 5), () {
           // Reuse code and reason from the local close.
           _closeCode = _outCloseCode;
           _closeReason = _outCloseReason;
-          _subscription.cancel();
+          if (_subscription != null) _subscription.cancel();
           _controller.close();
           _webSockets.remove(_serviceId);
         });
       }
     }
     return _sink.close();
   }
 
   void _close([int code, String reason]) {
     if (_writeClosed) return;
@@ skipping 38 matching lines @@
            (code < WebSocketStatus.NORMAL_CLOSURE ||
             code == WebSocketStatus.RESERVED_1004 ||
             code == WebSocketStatus.NO_STATUS_RECEIVED ||
             code == WebSocketStatus.ABNORMAL_CLOSURE ||
             (code > WebSocketStatus.INTERNAL_SERVER_ERROR &&
              code < WebSocketStatus.RESERVED_1015) ||
             (code >= WebSocketStatus.RESERVED_1015 &&
              code < 3000));
   }
 }