Adds a HomogeneousViewport class that works like MixedViewport but
handles only children that have all the same height.

Converts ScrollableWidgetList to use that, so that we don't waste a
frame looking at the size of the contents each time we change size.

This allows a number of seemingly pointless double-pumps in the tests
to be removed.

Other changes that were necessary to support the above:

 - RenderBlock now supports minExtent (think 'min-height' in CSS)
 - RenderBlock now supports itemExtent (forces the height of each
   child to be the same, so that the itemExtent passed to the fixed-
   height scrollables are all authoritative instead of a source of
   bugs when they don't match)
 - RenderBlockViewport now supports horizontal scrolling
 - improved the style of the isInfinite assert in box.dart
 - fixed the position of a comment in mixed_viewport.dart
 - added a test
 - made the logic for how many items to show be more precise

These constants aren't useful outside these files and are cause noise in the
dartdoc.

Extract install from StartSky

Update mixed_viewport example to new Key API

Also fixes #937.

Add an origin parameter to transforms

This parameter makes it easier to do math when you don't want to center your
tranform at (0, 0).

Avoid crashing when an iOS device isn't connected

Provide a fast path for MultiChildRenderObjectWrapper.syncRenderObject() when the children lists are identical.

Add dartdoc for the painting code

We were setting the main axis extent to zero because we had a sign error.

Fixes #918

BoxDecoration's borderRadius should be able to handle a value of double.INFINITY

If there are no other gestures in the arena, we should kick off the scroll
gesture right away. This change pulled a refactoring of how we dispatch events
to Widgets. Now we dispatch events to Widgets interleaved with their associated
RenderObjects. (Previously we dispatched to all of the RenderObjects first.)

We need the new version because of the TextPainter rename.

Also, add asserts that the text has layout before being painted.

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

Fixes #939

Disambiguate horizontal and vertical scrolling

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

Don't crash when no Android device is attached

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

Fix crash when a MixedViewport is initially offset at a non-zero position

add/remove/add smoketest
basic vertical behaviour
basic horizontal behaviour

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.

Improvements to demo game

...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.