docs: function resolve_(element, resolve) {
      // if element is a fragment, it will load everything
      // up to child element
      if (element.MaterialFragment) {
        element.MaterialFragment.loaded.then(() => {
          slice.call(element.querySelectorAll(selClass))
            .forEach(element => componentHandler.upgradeElement(element));
          resolve();
        });
      } else {
        element.addEventListener('load', resolve, { once: true });
      }
    }