Also, fix some minor bugs with SynchronousFuture.

* MultiTapGestureRecognizer previously would assert if there was no
   competition.
 * GestureArenaTeam would always select the first recongizer as the
   winner even if a later recognizer actually accepted the pointer
   sequence.
 * debugPrintStack would fail a type check if maxFrames was non-null.
 * FractionalOffset.lerp would throw a null-pointer exception if its
   second argument was null.

Also, add a number of tests for previously untested lines of code.

Also, make the exception handling for global key listeners slightly more
robust.

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.

`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.