This enum doesn't make sense anymore now that we can arbitrarily colorize
icons.  Instead, we just use a Color, which is both simpler and can be
interpolated during animations.

Fixes #1279

Makes it easier to use without a SizeObserver

Rather than managing all the Material Design icons manually, we now
manage them using an icon font. The icon font contains glyphs for each
icon in an efficient vector format.

This patch updates the FLX tooling to include the MaterialIcons font and
updates the Icon widget to use the font instead of asset images.

Fixes #2313
Fixes #2218
Fixes #2009
Fixes #994

This concept is now private to AnimationController. All the clients actually
want the AnimationStatus.

Fixes #1875

We originally wrote examples/fitness to be an app which all
members of the Flutter team could carry on their phones and
use every day.  It served us well for testing of Text input,
Keyboards and writing/reading JSON.  It's never actually become
a real carry app, so lets just let it go for now.

@abarth @collinjackson

This allows us to adjust exactly which gestures we're listening for
during layout, which I'll use to kill a SizeObserver.

Now we just have one TextSpan class that handles both simple strings, trees of
children, and styling both. This approach simplifies the interface for most
clients.

This patch also removes StyledText, which was weakly typed and tricky to use
correctly. The replacement is RichText, which is strongly typed and uses
TextSpan.

In the dark theme, there isn't really a primary swatch, so this API was
a sandtrap. Instead, be explicit about the colors we need for various
widgets in the theme.

Fixes #1277

This helper makes it easier to write correct tests that involve
gestures.

Fixes #1855

The example in the spec with multiple actions is an anti-example.

Fixes #1876

This object represents a rect the auto-layout system.

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.

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.

We weren't listening to the onChange handler.

Fixes #1850

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