Switching the selected index while the previous transition is still in progress is broken

[khorimoto-zz]

0. Use a CSS-based transition with a sufficiently long transition time, say 1 second.
1. Change the selected index, and the animation starts.
2. Within the 1 second of the transition, change the selected index back to the original index.

Expected: The previous transition force-finishes early, and the new one starts.
Actual: The first transition does not finish early, and the new property gets applied in such a way that it does not cause a transition at all. The transitionend event is not fired, so the transition does not finish until the 5-second timeout.

Note that if this is the intended behavior, then we should expose complete() as a public function so that clients can force-complete the previous animation if necessary.