1. We would assert if you tried to start an animation from within an
animation status callback. This is a common pattern, so I fixed this
assert (in Ticker._tick).
2. We would assert for animations with duration under a millisecond.
Fixed.

Also removed the workarounds assocated with #1.

It turns out that we aren't really using these. The identity logic is sufficient.

Also, add some asserts for a crash I had once but couldn't reproduce, in case that helps catch it next time.