doWrite(data, fn) {
        const req = this.request({
            method: "POST",
            data: 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.
@api private