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.

Fix some intrinsic constraints contract violations.

RenderBlock wasn't constraining the results.
RenderPadding wasn't constraining the results (which matters
especially when the constraints can't fit the padding in the first
place).
RenderViewport wasn't constraining the results.

Add a test for the block case.

To catch this kind of thing in the future, add some asserts to
debugDoesMeetConstraints() that all four intrinsic functions return
values that are within the constraints.

RenderBlockViewport doesn't support returning intrinsics, so turn off
the "no intrinsic support" asserts (and return zero) when we're doing
this new assert.

This new assert screwed up the custom layout classes' tests, so adjust
those tests to ignore the callbacks invoked from these asserts.

Add to the _debugReportException() method a short summary of the
descendants of this node. It's important to have this information when
debugging errors like these intrinsic constraints contract violations
because often nodes just pass the values through to their child so you
have to go several steps down to find the actual problem.

Fixes https://github.com/flutter/flutter/issues/1210

More elaborate RenderBox example

Reimplement the theme transition animation by actually animating the Theme