We only use them for the transition to dismissed.

Also add intervals and curves to AnimationPerformance, which affect all
variables used by the performance.

... and use them to improve the horizontal_scrolling example.

Now ScrollableBlock can combine a horizontally scrolling viewport with a
horizontal block.

Also rename ViewportScrollDirection to just ScrollDirection for less verbosity.

We'll need this for horizontally scrolling lists.

This is just one of many steps in preparation for turning
fitness into a play-store ready app.

@abarth @collinjackson

restart Sky apps on Android without having to restart SkyShell.

We still need to tune the various parameters, but this patch is a start.

I can't really make further progress until at least:
https://github.com/dart-lang/dartdoc/issues/766
is resolved, ideally also:
https://github.com/dart-lang/dartdoc/issues/763

@abarth

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

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.

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

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

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.

Refactors the sprite matrix handling to do most calculations in Dart, minimizes the number of calls down to Skia

This file was deleted.

Now you can import 'package:sky/widgets.dart' to get all the widgets.

* Intentions => Behaviors
* SlideIn => SlideTransition
* FadeIn => FadeTransition
* removed unused AnimatedContainer intentions

Simplifies the code a bit.

 - Center the labels on the tabs
 - Make the drawer's mask visible