code: function () {
                  // normalize IE9 bug (http://bugs.jquery.com/ticket/1450)
                  var status = _xhr.status === 1223 ? 204 : _xhr.status;
                  var body = null;
                  // HTTP 204 means no content
                  if (status !== 204) {
                      // responseText is the old-school way of retrieving response (supported by IE8 & 9)
                      // response/responseType properties were introduced in ResourceLoader Level2 spec
                      // (supported by IE10)
                      body = (typeof _xhr.response === 'undefined') ? _xhr.responseText : _xhr.response;
                      // Implicitly strip a potential XSSI prefix.
                      if (typeof body === 'string') {
                          body = body.replace(XSSI_PREFIX, '');
                      }
                  }
                  // fix status code when it is 0 (0 status is undocumented).
                  // Occurs when accessing file resources or on Android 4.1 stock browser
                  // while retrieving files from application cache.
                  if (status === 0) {
                      status = body ? 200 : 0;
                  }
                  var headers = Headers.fromResponseHeaderString(_xhr.getAllResponseHeaders());
                  // IE 9 does not provide the way to get URL of response
                  var url = getResponseURL(_xhr) || req.url;
                  var statusText = _xhr.statusText || 'OK';
                  var responseOptions = new ResponseOptions({ body: body, status: status, headers: headers, statusText: statusText, url: url });
                  if (baseResponseOptions != null) {
                      responseOptions = baseResponseOptions.merge(responseOptions);
                  }
                  var response = new Response(responseOptions);
                  response.ok = isSuccess(status);
                  if (response.ok) {
                      responseObserver.next(response);
                      // TODO(gdi2290): defer complete if array buffer until done
                      responseObserver.complete();
                      return;
                  }
                  responseObserver.error(response);
              }
docs: load event handler