Adds optional rect property to sprite layer class

Use ImageResource instead of Future<sky.Image>

The IgnorePointer class enables one to cut a widget subtree off from pointer events. This is useful when a sibling should not shield pointer events from overlapping siblings below it.

Added a ScrollListener listener to Scrollable. The ScrollListener runs each time the Scrollable's scrollOffset changes. This can be used to keep overlay widgets in sync with a Scrollable below them.

Removed the Scrollable ScrollClient API. It was no longer used and was clumsy to use as a ScrollListener.

Added global function findScrollableAncestor() to scrollable.dart.

Added examples/widgets/overlay_geometry.dart. The app's Scaffold is contained by a Stack. The Stack is used to display green overlay "Markers" at the corners of the most recently selected list item and where the corresponding tap occurred. The app uses widget.localToGlobal() to compute the global overlay positions of the markers. The ScrollListener is used to keep the markers' positions up to date.

Remove non-working Height setting

Fixes https://github.com/domokit/sky_engine/issues/547

@collinjackson

Fixes https://github.com/domokit/sky_engine/issues/548

@collinjackson

Using ImageResource solves two problems:

1) Listeners can be notified synchronously when the sky.Image is already
   available. This change removes flash of 0x0 layout when moving an
   already-cached image around in the render tree.

2) In the future, when we support animated images, we can notify listeners
   multiple times whenever a new image is available.

Fix typo in feed icon in drawer

Fixes https://github.com/domokit/sky_engine/issues/540

@abarth

Remaining pubspecs

Hello world pubspec

Address book pubspec

Simplify SkPicture

This patch simplifies the SkPicture we generate for Skia. Instead of drawing
everything into a nested SkPicture, we now draw everything into the top-level
picture, which requires us to apply the device scale factor in Dart.

Move main.dart to lib/main.dart for the game

This required me wrapping the new Activity APIs
as well as adding the missing pubspec.yaml

@abarth

Adds new Layer class to sprites

Fix tab indicator animation so it doesn't snap to the previous tab.

If you tap multiple tabs in a row, the tab animation used to snap to the
last selected tab when starting a new animation. Fix that.

Also use the BuilderTransition so we don't have to rebuild the tab bar
every frame.

Minedigger pubspec

Optimize circular clips slightly

This patch avoids creating weak references on every paint call by caching the
Path object between paints.

Just infer it from the sign of the velocity. That's what odeon does.

Move examples to the root

Fix asserts related to image drawing.

There were two problems:

1) When starting the mimic, we put up a bad frame because although we set the
   `begin` value of the animation, we were building using the current `value`,
   which hadn't been updated.

2) When stoping the mimic, we'd dirty a component during didUnmount, which
   wouldn't get cleaned until the next frame. Now we're sure to clean all the
   components before leaving flushBuild.