Is there an existing issue for this?

• I have searched the existing issues
• I have read the guide to filing a bug

Use case

So I noticed this after replacing PopupMenuButtons and DropdownButtons in our app with MenuAnchors and DropdownMenus.

According to the material design guidelines, there should be a fade transition when menu enters and exits on screen, but currently MenuAnchor just opens and close sthe menu without any animations.

Proposal

It's not a big deal, and would actually be preferable if on desktop and web, but on Android it kinds of look weird for the menus to just suddenly appears.