code: function () {
              var self = this, c = self.io.config, uri = c.uri, hostname = uri.hostname, iframe, iframeUri, iframeDesc = iframeMap[hostname];
              var proxy = PROXY_PAGE;
              if (c.xdr && c.xdr.subDomain && c.xdr.subDomain.proxy) {
                  proxy = c.xdr.subDomain.proxy;
              }
              if (iframeDesc && iframeDesc.ready) {
                  self.nativeXhr = XhrTransportBase.nativeXhr(0, iframeDesc.iframe.contentWindow);
                  if (self.nativeXhr) {
                      self.sendInternal();
                  } else {
                      LoggerManager.error('document.domain not set correctly!');
                  }
                  return;
              }
              if (!iframeDesc) {
                  iframeDesc = iframeMap[hostname] = {};
                  iframe = iframeDesc.iframe = doc.createElement('iframe');
                  Dom.css(iframe, {
                      position: 'absolute',
                      left: '-9999px',
                      top: '-9999px'
                  });
                  Dom.prepend(iframe, doc.body || doc.documentElement);
                  iframeUri = {};
                  iframeUri.protocol = uri.protocol;
                  iframeUri.host = uri.host;
                  iframeUri.pathname = proxy;
                  iframe.src = url.stringify(iframeUri);
              } else {
                  iframe = iframeDesc.iframe;
              }
              Event.on(iframe, 'load', _onLoad, self);
          }
docs: get nativeXhr from iframe document not from current document directly like XhrTransport