We were marking the parent *Widget's* render object as needing layout, but
we actually need to walk up the component tree until we find the parent
*RenderObjectWrapper* so we can mark actually mark the parent render object as
needing layout.

Fixes #408

Fixes anchor points for particles in particle systems

Make Switch a standalone class with a custom RenderObject

This changes Switch from being a subclass of the Toggleable widget to
being a standalone Component with a custom RenderObject. This is to
enable adding Switch-specific radial reaction animations in a
subsequent patch. The CustomPaint logic that Toggleable was using does
not provide a nice way for a class and its subclass to both participate
in deciding to repaint.

Combine all the scrollable classes into scrollable.dart

We ended up with a bunch of different scrollable widgets and it was confusing.
This CL moves them all into one library to make them easier to find.

Bump versions

Fix Dart analyzer warnings

@abarth

RenderStack should handle top, right, bottom, left all being zero

Sporadic crash from invalid touch event

This required some changes to AnimationPerformance to better understand
animating with forces.

Previously, we didn't set the width of the child correctly when top and bottom
(or left and right) were both specified.

Fixes #275

When embedded by the view_manager, sometimes we receive pointerup or
pointercancel events without having received a cooresponding pointerdown event.
The underlying issue is that the view_manager doesn't capture on pointerdown
and instead performs a new hit test for every pointer event. We should fix that
in view_manager, but, in the meantime, this patch makes us not crash in this
scenario.

Fixes #339

I incorrectly thought we were now supposed to import these mojoms from the
packages that publish them. However, we're still supposed to import them from
the virtual mojom package.

Also, update imports to account for new mojom.dart locations.

TBR=abarth@google.com

Adds star field to game start screen