     'vaadin-grid-sorter'
      ].forEach(elementName => {
        var element = this.querySelector(elementName);
        if (element && !(element instanceof Polymer.Element)) {
          console.warn(`Make sure you have imported the required module for <${elementName}> element.`);
        }
      });
    }

    /** @protected */
    _updateFirstAndLastColumn() {
      Array.from(this.shadowRoot.querySelectorAll('tr')).forEach(row => this._updateFirstAndLastColumnForRow(row));
    }

    /**
     * @param {