_mergeNewItems(items) {
    const children = Array.from(this.element.children);
    return sorter(this.items.concat(items), {
      by(element) {
        return children.indexOf(element);
      },
    });
  }
Combine the current items array with a new one and sort it by DOM order.
@param {ShuffleItem[]} items Items to track.
@return {ShuffleItem[]}