code: public function dropDownListDefault($items, $options = [])
      {
          Html::addCssClass($options, 'browser-default');
          return parent::dropDownList($items, $options);
      }
docs: Renders default browser drop-down list
  @see http://materializecss.com/forms.html#select
  @param $items
  @param array $options
  @return $this