 can be made.
       */
      items: {
        type: Array,
        readOnly: true,
        notify: true,
        value: function() {
          return [];
        }
      },

      /**
       * The set of excluded elements where the key is the `localName`
       * of the element that will be ignored from the item list.
       *
       * @default {template: 1}
       */
      _excludedLocalNames: {
        type: Object,
        value: function() {
          return {
            'template': 1
          };
   