We just need to enforce some finite clamp and Skia will do the rest.

Fixes #939

We now have separate gestures for horizontal, vertical, and pan scrolling.

Refactors a bunch of Android-related things into an AndroidDevice singleton class.

We need to always remove the widget when you sync a non-visible
widget, even if we already have it, because otherwise we'll try to
sync it with null again later, which causes a crash.

Test in #938.

...and not when we mount and dismount.

Turns out that when we dismount, it's too late -- we've already set
renderObject to null.

We also mark the mixed viewport as dirty when it is removed from its parent.
Without this, we try to reuse the child nodes in subsequent syncs, which is a disaster.

Instead, we pump a onScrollUpdate with the offset if there is an offset.

This patch replaces the scroll gestures used by Scrollable with ones detected
by the ScrollGestureDetector.

Add a way of having keys based on numeric types or DateTimes by having a ValueKey<T> class.
Remove the redundant ways of declaring things, except for leaving one shorthand -- you can say `new Key(s)` instead of `new ValueKey<String>(s)`.

Stop exporting framework.dart from basic.dart, since we now have widgets.dart exporting all of framework.dart.

Also clean up some formatting issues.

We're now using it at the widget layer for everything except scrolling and
flinging.

These gestures use the new gesture detection system.

In the GestureArena, gestures will fight it out to see which is the real
ultimate gesture.

Previously, if you used a layout callback, you could not have a parent
that did parentUsesSize, or if you did, you had to be marked
sizedByParent. With this patch, we allow the parent to depend on your
layout, even if you modify your child list during your layout using a
layout callback, by checking that the parent is still actively being
laid out in this scenario.

This patch is the first step towards implementing gestures. The pointer router
allows the gesture detectors to hook in at the end of the pointer event
propagation chain.

Related to #145

@abarth

Also includes changes to widget_tester sufficient to
test scrolling.

@abarth

Have “sky_tool listen” start with a refresh so that iOS devices don’t wait on a black screen for the first filesystem change.

Previously, once a callback was in the execution queue, it couldn't be
canceled. Now we check whether the callback was canceled before executing it.

For debugging #801

The core of MultiChildRenderObjectWrapper.syncRenderObject() could
apply to any subclass that uses a flat child list, so this abstracts
it out into the superclass.

(Also, instead of requiring the callbacks of RenderBlockViewport to
constrain their results, we just constrain it for them. Makes things a
bit easier for users of that class.)

Added some unit test infrasture for checking layers.

- add debugDescribeSettings to a few classes that were missing it
- fix some minor bugs in RenderShrinkWrapWidth and ShrinkWrapWidth
- introduce RenderShrinkWrapHeight and ShrinkWrapHeight

Generalise RenderBlockViewport so that it can be used by a Widget that knows its children's dimensions.

I'll write the Widget wrapper in the next CL, including adding
support for padding at the Widget layer.

@Hixie

Most of this work in this patch is building out the test framework to the point
where we can write this test.

Fixes #454.

This hopefully explains things well enough that it fixes #846, after a manner.

This still leaves Flex and FlexDirection available. At some point once
people have transitioned to Row/Column we should rename Flex to _Flex
and stop reexporting FlexDirection from basic.dart.