This requires all AnimationController objects to be given a
TickerProvider, a class that can create the Ticker.

It also provides some nice mixins for people who want to have their
State provide a TickerProvider. And a schedulerTickerProvider for those
cases where you just want to see your battery burn.

Also, we now enforce destruction order for elements.

This also required changing the AnimationController state transition
logic to signal completion of the animation during the tick that
finishes the simulation.

Fixes https://github.com/flutter/flutter/issues/3675

`flutter test` now has a `--merge-coverage` flag that can be used to merge
coverage data from previous runs, enabling faster iteration cycles.

We now have 100% coverage of animation.dart and animation_controller.dart.
Also, add some basic tools for working with lcov files. These tools need much
more polish.

Also, fix the bug where apps would assert when you change the time dilation.
Use that mechanism to fix the animation controller tests.

Instead of flinging to between 0.0 and 1.0, we should adapt the default
spring to the controller's upper and lower bounds.

Fixes #3545

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