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.

As of `1.18.0-dev-0`, these cases will get flagged.  In the meantime, the

Also, add more dartdoc to widgets.dart.

We used to use this mechanism to paint scrollbars, but it's awkward. The
new approach to scrollbars is much cleaner. This mechanism has no other
clients, so we should remove it.

This patch updates the date picker to match the new spec.

We're still missing a fade effect when scrolling from one month to
another and we're missing the landscape layout.

Fixes #3558

* Remove ScrollableListPainter

LazyBlock is intended as a replacement for MixedViewport. Rather than

maintaining a table of all the observed child sizes (like

MixedViewport), LazyBlock works by dead reckoning the location of the

children based on the existing viewport. This approach makes it easier

to resize children because LazyBlock doesn't cache any additional

information that would need to be invalidated.



This patch contains a first draft of LazyBlock that works in a simple

usage scenario. Subsequent patches will replace

ScrollableMixedWidgetList with LazyBlock and port the existing

ScrollableMixedWidgetList tests over to LazyBlock.



Related to #3075

Previously, when the content extent changed during a scroll interaction, we'd
stop the current scroll interaction and reset the scroll offset. Now we try to
continue the scroll interaction (e.g., drag, fling, or overscroll) even through
the underlying scroll behavior has changed.

For physics-based scroll interactions, we keep the current position and
velocity and recompute the operative forces. For drag interactions, we keep the
current position and continue to let the user drag the scroll offset.

After this patch, we still disrupt non-physical scroll animations that are
operating outside the new scroll bounds because it's not clear how we can
sensibly modify them to work with the new scroll bounds.

Fixes #2448

Fixes #774

Fixes #581

And fix the zillion issues that uncovered.

This reverts commit f41b3411, reversing
changes made to e33d8d96.

This was a bad check-in due to my mangling uploading a new version of the branch from a different machine.

This reverts https://github.com/flutter/flutter/pull/2639 and will be replaced by https://github.com/flutter/flutter/pull/2640

And fix the zillion issues that uncovered.

We'll need this for RTL support because the RTL state will live in the widget
tree. Also, remove the `oldWidget` argument to updateRenderObject because there
aren't any clients for it.

Remove the SizeObserver you need to use snap alignment in lists by
having the lists provide the size themselves in the callback.

Also, remove snapAlignmentOffset since we couldn't figure out how to
explain it and it's not really needed.

Each layer is supposed to reexport the parts of the previous layer
that are part of its API.

- In painting.dart, export from dart:ui all the Canvas-related APIs
  that make sense to be used at higher levels, e.g. PaintingStyle.

- Delete painting/shadows.dart. It was dead code.

- In rendering/object.dart, export all of painting.dart.

- In widgets/basic.dart, export all of painting.dart and
  animation.dart. Some classes in animation/ are renamed to make this
  less disruptive and confusing to the namespace.

- Split out Stocks back into an import model rather than a part model,
  so that it's easier to manage its dependencies on a per-file basis.

- Move Ticker to scheduler library.

- Remove as many redundant imports as possible now.

- Some minor nit picking cleanup in various files.

The virtual viewport machinery now handles all of these use cases.
Previous clients of ScrollableWidgetList can use ScrollableLazyList
instead.

Previously this code was in the animation layer, which didn't make much
sense because scrolling is a widget concern.

We were recomputing which widgets to show only when we were on the other side
of the repaint boundaries. That doesn't work well for pageable lists because we
come to rest exactly on a repaint boundary, which means we don't cull the other
page.

After this patch, we recompute the set of widgets using an edge-trigger when we
hit the boundary. That's better than using a level-trigger so that we don't
continuously recompute the set of widget as we sit at the boundary.

We use the ScrollDirection for more than just scrolling.

Fixes #151

This patch moves PageableList off HomogeneousViewport and onto RenderList and
friends, making it match the new ScrollableList.