Split out Instrumentation logic from WidgetTester

This will allow writing tests/benchmark which want to use
the engine's default beginFrame and normal passage of time.

@Hixie

Always import our packages when analyzing.

Only set tolerance if simulation is non-null

Fixes #898

Update SoundEffectPlayer to use the SoundPool API

Tabbar view swipe

Swiping left or right in a TabBarView now changes the selected tab in the way it's supposed to.

Currently swipe gestures that start while the selection change is underway are ignored. Will fix that in a separate change.

Swiping left or right in a TabBarView now changes the selected tab in the way it's supposed to.

Currently swipe gestures that start while the selection change is underway are ignored. Will fix that in a separate change.

Analyze more tests.

Use Scheduler.instance in flutter_sprites

support mini fabs

Rename Pointerer to Gesturer.

Refactor bindings.

Avoid rebuilding entire app when an overlay needs updating

Use hashValues instead of hard-coded hashCode getters

rev the engine

Minor fixes in comments and style.

Upgrade the test package (needed by the latest mojo roll)

applyPaintTransform() improvements

Previously, applyPaintTransform() had to know how it was positioned in
its parent, even though that's really the parent's responsibility.

Now, applyPaintTransform() is given a child and applies the transform
that it would give the child during paint.

This makes it possible for applyPaintTransform() to work across
coordinate system boundaries (e.g. box to sector, or view to box --
previously, view to box only worked because we explicitly skipped that
step -- since view doesn't actually apply a transform, it doesn't
really matter).

Check for cycles in our various tree structures.

Add dartdoc for image classes

 - Adds dartdoc for all the `of` functions.
 - Renames Image to RawImage. This widget is rarely used and shouldn't take up
   such a nice global name.

Fixes #361

Remove reference to scheduler in animation.

Add more dartdoc for basic widgets

Added rotation test for new PageableList code.