Async File Uploading in Dart

async_file_upload.dart

void handleUpload() {
  var elem = query("#upload") as FileUploadInputElement;
  var file = elem.files.first;

  FormData fd = new FormData(null);
  fd.append("username", "ajlai");
  fd.appendBlob("Filename", file);

  var req = new HttpRequest();
  req.open("POST", "/foo");
  req.onReadyStateChange.listen((_) {
    if (req.readyState == 4 && req.status == 200) {
      // callback here
    }
  });
  req.send(fd);
}