Protect against infinite recursion in focusOverlay

[nevir]

An easy way to repro the infinite recursion is to create multiple opened overlays, and dismiss them at the same time (toasts).

Here's what's happening. Say there are two overlays (A, and B), that were dismissed at the same time - and that their transitions end within the same run loop tick:

• A.transitionend:
  • Removed from overlays.
  • Calls applyFocus, which calls focusOverlay.
• focusOverlay calls B.applyFocus:
  • B.closed == false, so focusOverlay is called again.
  • ...recurse.

The call to async lets the overlays stack resolve before showing the next one.

[nevir]

Ack, there's more to this - sometimes overlays aren't being removed when there's a stack (transitionend isn't firing). I need to dig further.

Closing until I've got a better idea of what's going on.

[nevir]

Sorry about the flapping on this issue; but here's a more resilient fix:

• core-overlay now tracks when it's being transitioned (would that be better as state on core-transition?)
• It no longer attempts to focus/blur overlays that are mid-transition.

[jlg7]

+1 - Stumbled onto same problem this afternoon when trying to use paper-toast and core-overlay in conjunction. thanks @nevir

[sorvell]

LGTM