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

I removed these functions by mistake in my previous patch.

When we start, we might not have initialized the devicePixelRatio value. The
value will be initialized by the time we paint, so now we update the value
every time we paint.

Use sky.SceneBuilder to upload Layer tree to C++

We still use SkPicture as our rasterization backend, but now we're uploading
our layer tree to C++. A future patch will push the layer tree deeper into the
system and we'll eventually switch backends.