To match the Align widget (and several other widgets).

Fixes #6101

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.

See #4434.

* Added OverscrollIndicatorEdge et al

* RefreshIndicator only clamps its scrollable edge

* added a test

* Updated the test

* fixed lint-os

* fixed a typo

* Scrollable should restore its viewport dimensions when it reappears

* removed an accidental commit

* updated per review feedback

These let you add Align widget to the inside or outside of a container.
Several customers have asked for these properties.

Fixes #4950

Making progress towards document all public APIs in material.dart. We're still
missing a few odds and ends (as well as missing docs in tabs.dart,
tooltip.dart, and two_level_list.dart).

This patch completes the circuit on this library. The next step is to go
through and add docs to anything we missed.