Add device pixel ratio to MediaQuery

Don't autofocus Input widgets by default

Remove unused close method from AssetBundle

Instead, require the developer to opt-in to autofocusing because autofocusing
can be disruptive.

Fixes #1307

Scaffold should respect window.padding.bottom

The space for the keyboard is now represented as bottom padding for the window.
By teaching the scaffold to respect the bottom window padding, we move the
floating action button and snackbars out of the way of the keyboard.

This currently works on Android. I'll need to see how to get the keyboard
geometry on iOS for a similar effect.

Fixes #103

refactor commands to move out re-usable functionality

Move scroll_behavior into widgets

Previously this code was in the animation layer, which didn't make much
sense because scrolling is a widget concern.

parse out the ios simulator device name

Minor animation renames

These should be the last references to the old "performance" name.

Some of the Animation classes that we converted from performances use
the term "master" to refer to the animation upon which they're based.
This patch changes them to use the term "parent", which is consistent
with the rest of the animation classes.

Make `flutter start` not report erroneous failure

Was failing when starting in the iOS Simulator

@chinmaygarde

Merge simulationStepper into AnimationController

Now that we've decided that Animation<double> isn't confined to the interval
0.0 to 1.0, we can expand AnimationController to cover the use cases that used
to require SimulationStepper.

This patch merges SimulationStepper into AnimationController and ports the one
stand-alone client of simulation stepper over to using AnimationController.

Add section images to the gallery home page

Move ClampedSimulation into newton

refactor build command into two files

ClampedSimulation makes more sense at the newton layer because it's dealing
entirely with newton concepts.

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