Don't propagate http client error, when no response object is available.

runtime/bin/http_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.
 
 class _HttpRequestResponseBase {
   _HttpRequestResponseBase(_HttpConnectionBase this._httpConnection)
       : _contentLength = -1,
         _keepAlive = false,
         _headers = new Map();
 
@@ skipping 914 matching lines @@
     _httpParser.dataReceived = (data) => _onDataReceived(data);
     _httpParser.dataEnd = () => _onDataEnd();
     _httpParser.error = (e) => _onError(e);
     // Tell the HTTP parser the method it is expecting a response to.
     _httpParser.responseToMethod = _method;
 
     onDisconnect = _onDisconnected;
   }
 
   void _propagateError(Exception e) {
-    if (_response._streamErrorHandler != null) {
+    if (_response != null && _response._streamErrorHandler != null) {
       _response._streamErrorHandler(e);
     }
   }
 
   HttpClientRequest open(String method, String uri) {
     _method = method;
     _request = new _HttpClientRequest(method, uri, this);
     _request.keepAlive = true;
     _response = new _HttpClientResponse(this);
     return _request;
@@ skipping 251 matching lines @@
     sockets.addFirst(socketConn);
     socketConn._markReturned();
   }
 
   Function _onOpen;
   Map<String, Queue<_SocketConnection>> _openSockets;
   Set<_SocketConnection> _activeSockets;
   Timer _evictionTimer;
   bool _shutdown;  // Has this HTTP client been shutdown?
 }