- Make space for the iOS status bar.
 - Set the iOS status bar to use white text.

Make Sky apps use the full screen on iOS.

Initial version of game performance tests

Fix RenderViewport's hit testing

Updated expected.txt files for stocks, tabs

We were subtracting the scrollOffset when we were supposed to add it.

Improve scrollable tabs performance

Fix skydoc to work again

@abarth

Remove unused CSSMatrix files

When animating, use the same curve until it completes.

This ensures we don't run into discontinuities when reversing an
animation halfway through. I refactored AnimationValue to have knowledge
of the reverse curves and intervals.

Rename widgets/widget.dart to widgets/framework.dart

It was confusing to have both widget.dart and widgets.dart

Make sky_tool much more user-friendly.

Switch the default whitespace mode to pre-line

This mode will preserve new lines, allow breaks, but collapse whitespace. That
means whitespace at the start of lines will be collapsed.

Add RenderToggleable base and use in Switch and Checkbox

This refactors Checkbox to own a RenderObject similar to how Switch was
refactored in https://github.com/domokit/sky_engine/pull/376 and
extracts common functionality for toggleable renderers into a base
class.  Switch and Checkbox's render objects derive from this base
class to add their own custom painting and theming logic.

B=Fixes #187

First pass on sprite 3D node

Fixes path in SerializePicture call

Optimizes drawing performance in sprites

Enable GrLayerHoister optimizations in Skia

Turns out we weren't passing kComputeSaveLayerInfo_RecordFlag, which meant we
weren't computing the information necessary for the GrLayerHoister in Skia to
do its work.

This CL makes demo_launcher scrolling 3x faster on a Nexus 5.

Teach event system about disposition and make 'consumed' disposition …

Refactors the sprite matrix handling to do most calculations in Dart

This introduces the notion of event disposition and allows event
targets (widgets and render objects) to consume events that should not
be processed further. This is needed by the Switch component in the
Drawer in the stocks example. The Switch is embedded in a DrawerItem.
The Switch handles the gesture tap event to toggle its state and should
handle pointer events to allow swiping and draw its own radial
reaction. The DrawerItem also handles gesture taps to allow toggling
the switch value when tapping anywhere on the drawer and to draw its
own ink splash. When tapping on the switch, both the switch's render
object and the DrawerItem's listener are in the event dispatch path.
The Switch needs to signal in some fashion that it consumed the event
so the DrawerItem does not also try to toggle the switch's state.