Simplify the AutoLayout API

This object represents a rect the auto-layout system.

fixes found when running through the getting started process

This patch makes it easier to use the auto layout API:

* We no longer use operator== because that requires an ugly cast by the
  API user.
* Also, "leftEdge" is now just "left" for less verbosity.
* AutoLayoutChild not implies its key from the AutoLayoutParam object.
* We now correctly layout every child of a RenderAutoLayout object even
  if the solver doesn't flush any updates to that child.

Make update_dart_sdk.sh prettier and more resilient

Mention that cache directory is volatile.

Add support for autolayout to widgets

Internalize our dependency on the Dart SDK

This patch teaches the widget framework how to use Cassowary-based
autolayout. To integrate autolayout with widgets, I had to refactor how
RenderAutoLayout worked a bit. Now RenderAutoLayout follows the same
delegate pattern we use for custom paint and custom layout.

This patch teaches the flutter command to download the Dart SDK into
cache, removing another step from the Getting Started workflow. The
version of the Dart SDK is controlled by the "dart-sdk.version" file in
the bin/cache directory.

Fixes #54

Improve checkbox animation

This patch improves the checkbox animation as suggested by the design
team. The color transition now occurs earlier in the animation and there
are fewer components to the animation.

Fixes #1614

Stocks input field for company name doesn't work

rename list->devices, start->run

We weren't listening to the onChange handler.

Fixes #1850

Document the onPressed/disabled behavior.

Fixes https://github.com/flutter/flutter/issues/1331

Hopefully this will be enough to start with. If this continues to be a
point of confusion we can see about adding more documentation or
changing the API somehow.

Rename debugDescribeSettings to debugFillDescription.

Add a Velocity class to be explicit about units

...so that it's easier to copy/paste between the rendering and settings layers.

We were using an Offset, which represented pixels/second, but it wasn't
clear to clients whether that was pixels/ms. Now we use a Velocity class
that is explict about the units.

Fixes #1510
Fixes #785

Fix infra docs

Previously we had the wrong path to the recipes.py script and an incorrect argument list.

Use Image.toString instead of rolling our own each time

Add a raw hello_world that shows "Hello, world"

I meant to check this in as part of https://github.com/flutter/flutter/pull/1852
but forgot to commit the local change, d'oh.

Fix color of icons in drawers in dark theme.

Disable flakey test.

See https://github.com/flutter/flutter/issues/1854

This makes it match the material spec more.
https://www.google.com/design/spec/style/icons.html

Fixes https://github.com/flutter/flutter/issues/1357

Previously, hello_world.dart was an interactive circle. I've moved that
to touch_input.dart. We should eventually harmonize the touch input
examples at all the layers.

Adds documentation to the layering examples

allow any android sdk version