mnElement);
    }

    /** @private */
    _flattenColumnGroups(columns) {
      return columns.map(col => {
        if (col.localName === 'vaadin-grid-column-group') {
          return this._getChildColumns(col);
        } else {
          return [col];
        }
      }).reduce((prev, curr) => {
        return prev.concat(curr);
      }, []);
    }

    /** @private */
    _getColumnTree() {
      var rootColumns = Polymer.FlattenedNodesObserver.getFlattenedNodes(this).filter(this._isColumnElement);
     