    * Returns if a element is focusable.
       * @param {!HTMLElement} element
       * @return {boolean}
       */
      isFocusable: function(element) {
        // From http://stackoverflow.com/a/1600194/4228703:
        // There isn't a definite list, it's up to the browser. The only
        // standard we have is DOM Level 2 HTML https://www.w3.org/TR/DOM-Level-2-HTML/html.html,
        // according to which the only elements that have a focus() method are
        // HTMLInputElement,  HTMLSelectElemen