I was looking for the same and found the work around as
1. Go to Appearance -> Menus
2. Go to top right corner for Screen Options & click open the pan.
3. Check on CSS Classes
4. Open the menu you want Dropdown (parent menu)
5. You will find a CSS Classes (optional)
6. Enter following text there has-flyout
7. Save the menu.
Enjoy your Dropdown menu.
Hope it works!
