code: function (url, cb) {
  
              var request = new XMLHttpRequest();
  
              request.onload = function () {
                  if (this.status >= 200 && this.status < 400) {
                      cb(this.response);
                  }
                  else {
                      Logger.log(Logger.LEVEL_SEVERE, "sever error: " + this.status);
                      cb(null);
                  }
              };
  
              request.onerror = function (e) {
                  Logger.log(Logger.LEVEL_SEVERE, "connection error: " + e);
                  cb(null);
              };
  
              request.open("get", url, true);
  
              request.send();
          }
docs: Fetches a file.
  @param {String} url The path to the collada file.
  @param {Function} cb A callback function to call when the collada file loaded.