I'll write the Widget wrapper in the next CL, including adding
support for padding at the Widget layer.

@Hixie

Make RenderBlockViewport shrink-wrap its children in the main axis direction

Add a basic test for DatePicker

Most of this work in this patch is building out the test framework to the point
where we can write this test.

Update the documentation on sizing, make another assert point to it.

Moves GameObjectFactory and PlayerState to their own files in demo game

Fixes #454.

Adds power ups to demo game

Turn big_circle.dart into a test

Fixes #829

Sector demo is broken because of compositor changes

We need to implement visitChildren for the RenderObjects in the sector demo.
Also, add a test.

Fixes #790

Fixes issues in sprite constraints

Add date picker to widgets library and teach fitness app to use it

This hopefully explains things well enough that it fixes #846, after a manner.

This still leaves Flex and FlexDirection available. At some point once
people have transitioned to Row/Column we should rename Flex to _Flex
and stop reexporting FlexDirection from basic.dart.

Add a basic test for Mimic tree movement

Also, add an example for the date picker

Change Fitness app id

This test just exercises Align to make sure it can sync.