TransitionDoneHandle

Handle for a pending onTransitionDone wait.

flush completes the wait immediately: it removes the listener, clears the fallback timer and invokes onDone (once - later calls and late events are no-ops). cancel tears the wait down silently: listener and timer go away and onDone never fires. Call cancel from a destroy hook so a directive torn down mid-transition does not receive a late onDone from the fallback timer.