doWrite(data, fn) {
    const req = this.request({
      method: "POST",
      data
    });
    req.on("success", fn);
    req.on("error", (xhrStatus, context) => {
      this.onError("xhr post error", xhrStatus, context);
    });
  }
Sends data.

@param {String} data to send.
@param {Function} called upon flush.
@private