How to use AJAX in Dart with async = true

There is an example at https://code.google.com/p/dart/source/browse/branches/bleeding_edge/dart/samples/belay/bcap/bcap_client.dart

  void privateInvoke(String ser, String method, String data,                     SuccessI ski, FailureI fki) {    if (urlRegex.hasMatch(ser)) {      var req = new XMLHttpRequest();      req.open(method, ser, true);      req.on.readyStateChange.add(void _(evt) {        if (req.readyState == 4) {          if (req.status == 200) {            ski(req.responseText);          } else {            fki(new BcapError(req.status, req.statusText));          }        }      });      req.send(data);      return;    } else {      super.privateInvoke(ser, method, data, ski, fki);    }  }

also another asynchronous example in https://code.google.com/p/dart/source/browse/branches/bleeding_edge/dart/client/samples/total/src/ServerChart.dart

XMLHttpRequest request = new XMLHttpRequest();request.on.readyStateChange.add((Event event) {  if (request.readyState == XMLHttpRequest.DONE && request.status == 200) {    callback("data:image/png;base64,${StringUtils.base64Encode(request.responseText)}");  }});...request.open("POST", url, true, null, null);request.setRequestHeader("Content-type", "text/plain");request.overrideMimeType("text/plain; charset=x-user-defined");print("Chart request: ${data}");request.send(data);