The `flutter` package gets more types!

Follow-up from: https://github.com/flutter/flutter/pull/3727.

The old names were getting silly and started stepping on valuable namespace.

The new names are consistent and clear.

Fixes #2324

Previously we would elide forward and reverse callbacks that canceled each
other out, which broke the expected state machine. Now we synchronously deliver
status callbacks when start an animation.

Fixes #1913

We should mutate the status of the AnimationController before calling
the status callbacks so that we're prepared to be re-entered.

Fixes #1911