Port the remainder of the framework to AnimationController

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.

There should be no more uses of Performance or AnimatedValue in the framework
or the examples.

Port most of widgets to AnimationController

I've left transitions and enter_exit_transition out of this patch, but I've
converted the rest.

Finish porting material.dart to AnimationController

After this patch, there aren't any direct uses of Performance in material.dart.

Device notification

name and grammar tweaks

Add support for running flutter tests on Chromium Infra

Move Scaffold over to using AnimationController

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

This patch moves Navigator and related code over to using
AnimationController.

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.

Better failure when run from non-flutter projects

fix an issue where errors were printed twice

Revised Material Gallery demo

The demo's home page is now a grid divided into the same six sections that appear in the Material Design spec itself. Tapping on a section leads to a list of demos. Currently only the "Components" section has any demos.

All of the demos are now self-contained apps with their own Scaffold and etc.

Port examples to the new animation API

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

First take on `flutter start` for iOS devices

add a wrapper around the adb command

Localize curve direction concern

The curve direction is now a concern of ACurve alone.

Introduce Tween and the new animation API

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

fix two null-aware-operator issues

StackParentData.toString() fix, RenderOpacity default

Move RenderObjectWidgets down

It's confusing to have non-render-object widgets on either side of the
render object ones.