md-menu-bar child menus don't stay open

[srcn]

When you hover over the child menus in md-menu-bar, they are not staying open.

To reproduce the issue:

1. Go to https://material.angularjs.org/latest/demo/menuBar
2. Click on the File menu
3. Hover over the New menu item
4. Try to go over the child menu and it will be closed

This only occurs when you open the menu for the first time. If you completely close the File menu and try it again, it works as intended.

Tested on Chrome 47 (Mac)

[jraadt]

@rschmukler - I've been looking through your latest menu commits in the last 2 days and it seems to be somewhere in there. Any ideas?

Also, if you have multiple submenus (for instance if "Open" had a submenu in the menuBar demo), then you need to open each one individually, close the entire menu, and re-open it for them all to work.