Method getFallback

src/custom-element.js:1962–1964

* Returns an optional fallback element for this custom element. * @return {?Element} * @package @final

()

     * @package @final
     */
    getFallback() {
      return query.childElementByAttr(this, 'fallback');
    }

    /**
     * Hides or shows the fallback, if available. This function must only