src/widget/Dropdown.php
public function init()
    {
        parent::init();
        if($this->type){
            $class = ($this->type == self::DROPDOWN) ? 'dropdown-menu' : 'treeview-menu';
            Html::addCssClass($this->options,$class);
        }else{
            Html::addCssClass($this->options, 'dropdown-menu');   
        }
    }
public function init()
    {
        parent::init();
        if($this->type){
            $class = ($this->type == self::DROPDOWN) ? 'dropdown-menu' : 'treeview-menu';
            Html::addCssClass($this->options,$class);
        }else{
            Html::addCssClass($this->options, 'dropdown-menu');   
        }
    }
Initializes the widget.
If you override this method, make sure you call the parent implementation first.