I see a sub-menu working in the "menu bar" example - https://material.angularjs.org/latest/demo/menuBar
There are no examples of sub-menus for the actual menu component.
I looked at the menu bar example and it uses md-menu, so it should be supported in the regular menu, right? I tried it and it breaks the entire menu (top level also stops working).
I'm guessing that it's just not supported? I found this discussion, but I'm not sure what to make of it.