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 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.

Moves sky sprites to its own package

Introduce Align widget for aligning children.

Properly track mimics that move in the tree

We weren't telling the new mimicable that we wanted to start mimicing it.

Updates to demo game

@abarth

Don't freeze devicePixelRatio at startup

We were making local copies of the listener maps, but we were actually
iterating the underlying sets. Now we make local copies of the sets.

Fixes #803