This patch removes Performance and AnimationValue now that we've ported the
framework over to AnimationController and Tween. This patch also cleans up the
names of the AnimationController classes now that they don't have to avoid
conflicts with the old animation API. Specifically, I've made the following
renames:

 * Animated -> Animation
 * Evaluatable -> Animatable
 * PerformanceStatus -> AnimationStatus

This patch is just renames and moving code around. There aren't any changes in
behavior.

Providing a pre-built child is more efficient because we don't need to rebuild
the child every tick of the animation.

Also, clean up the class hierarchy for AnimationController now that
we've renamed progress to value. That means everything in the hierarchy
now has a value, include Watchable. This patch renames Watchable to
Animated<T>, which lets us use that type almost everywhere.

I've added some ducktape to modal bottom sheets to avoid having to
refactor all of Navigator to use AnimationController. I'll remove the
ducktape in the next patch.

These now use Animation and AnimationController instead of PerformanceView and
Performance.

This patch removes state from the animation system, which was causing problems
as we were scaling the use of animated values.

Now the "tween" objects are stateless and can watch animations, which creates a
new object that holds both the tween and the animation instead of mutating the
tween every tick of the animation.

This patch ports one client as a proof-of-concept.

Fixes #215

Previously we used a positional argument for widgets that had multiple
children. Now we use a named argument that defaults to an empty list.

Fixes #241

This makes skyanalyzer also check the examples, and fixes everything it
found there.

Fixes #1669

These are now part of material.dart.

Rather than using a custom render object, we can just use a Stack.

This patch simplifies AnimationTiming and all the AnimatedValue base classes.
Also, make PopupMenu a stateless component because it has no state.

Fixes #1168

Fixes #1170

This argument isn't needed anymore now that State has a getter for context.

ComponentState becomes State, for brevity.
Instead of overriding its constructor, override initState().
This makes writing States much simpler.

This fixes #1103.

Also, introduce Colors and Typography to hold the material colors and the
typography declarations. Previously we expected clients of these libraries to
import them into a namespace, but that doesn't play nice with re-exporting them
from material.dart.

Move the animation libraries into src/animation and change importers to use
package:sky/animation.dart. Also, move scheduler.dart into the animation
library so that the animation library can be self-contained.

We're now using it at the widget layer for everything except scrolling and
flinging.

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.

Fixes #743

Similar to widgets.dart, rendering.dart exports the entire rendering layer.
Also, update the examples to use rendering.dart and widgets.dart. Also clean up
some exports so that the examples have more sensible imports.

We had a remarkable number of analyzer failures.

I'll fix the bots to analyze across the whole project
in a follow-up patch, that should prevent this
in the future.

@abarth

Setting a ProgressIndicator's value to null (the default) makes it an "indeterminate progress" or activity indicator.

The indeterminate animations for both kinds of progress bars are essentially the same and wrong vis the Material Design spec, http://www.google.com/design/spec/components/progress-activity.html. I'll improve conformity with the visual design in a future CL.