flush() {
    if (this.running) {
      return this.running;
    }

    if (this._q.length) {
      this.running = this.dequeue().then(function () {
        this.running = undefined;
        return this.flush();
      }.bind(this));
      return this.running;
    }
  }
Flush all, as quickly as possible
@return {Promise}