for the `iron-items-changed` event to respond to
     * updates to the set of selectable items after updates to the DOM list and
     * selection state have been made.
     *
     * WARNING: If you are using this method, you should probably consider an
     * alternate approach. Synchronously querying for items is potentially
     * slow for many use cases. The `items` property will update asynchronously
     * on its own to reflect selectable items in the DOM.
     */
    forceSynchronousItemUpdate: functi