Now IconThemeData.fallback is a factory constructor and IconThemeData.of() does
the work of computing the fallback for its clients.
Also, add tests for ImageIcon and ListItems.
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.
