This caught a couple bugs in the previous commit. First, I wasn't sending the
correct index (i) to the "end" event listeners, because it was being masked by
a local variable. Second, the transition lock was being incorrectly deleted when
a transition is inherited on the same node and there is zero delay and
d3.timer.flush is called. There was potentially a related bug where the active
field was not being cleared on the transition lock. Yay tests!
Mike Bostock