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