Add Diagnosticable base class and documentation

Refactor RenderObject.toStringDeep and Widget.toStringDeep code to use DiagnosticsNode.

Turns out we have many immutable classes.

Fixes #6892

Mostly related to semantics and scrolling.

This method seemed overly specific to the needs of one particular
subclass. This patch duplicates some code but makes the API conceptually
cleaner.

Fixes #8267

Fixes #7734

Rename all the "2" classes related to scrolling to remove the "2". Now that the
old scrolling code is gone, we don't need to use the suffix.

All the clients have migrated to Scrollable2.

This patch makes a number of changes to how you can configure a
Scrollable2:

 - The ScrollPhysics is now responsible for creating the ScrollPosition.
   You can override the ScrollPhysics by supplying a `physics` argument
   to `Scrollable`, and the new physics you supply will be applied to
   the default physics inherited from the ScrollBehavior.

 - This patch removes the ScrollPosition/AbsoluteScrollPosition split as
   all clients were operating in pixels anyway and the split made the
   code very difficult to follow.

 - ScrollPosition no longer depends directly on Scrollable2State.
   Instead, it depends on an abstract interface that Scrollable2State
   implements. This change has two benefits:

    a) It removes the circular dependency between ScrollPosition and
       Scrollable2State, which lets us split the code for these classes
       (and several other classes that got wrapped up in that cycle) into
       separate libraries for easier maintenance.

    b) ScrollPosition is no longer bound to Scrollable2, which means you
       could use the behavior machinery to drive other sorts of widgets.
       For example, we could use it to drive Scrollabe1 if we wanted.

This patch adds `@checked` everywhere is needed to remove the
`strong_mode_invalid_method_override` strong mode error.

* 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

Changes in this patch:
- iOS now uses a different scrollDrag constant than Android.
   - ScrollConfigurationDelegate now knows about target platforms.
   - ScrollBehaviors now know about target platforms.
   - RawInputLine now has to be told what platform it's targetting.
   - PageableList now has a concept of target platform.
- make debugPrintStack filter its stack.
   - move debugPrintStack to `assertions.dart`.
- add support for limiting the number of frames to debugPrintStack.
- make defaultTargetPlatform default to android in test environments.
- remove OverscrollStyle and MaterialApp's overscrollStyle argument. You
  can now control the overscroll style using Theme.platform.
- the default scroll configuration is now private to avoid people
  relying on the defaultTargetPlatform getter in their subclasses (since
  they really should use Theme.of(context).platform).
- fix some typos I noticed in some tests.
- added a test for flinging scrollables, that checks that the behavior
  differs on the two target platforms.
- made flingFrom and fling in the test API pump the frames.
- added more docs to the test API.
- made the TestAsyncUtils.guard() method report uncaught errors to help
  debug errors when using that API.

Also, add more dartdoc to widgets.dart.