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

These files are already in the "material" folder. They don't need to be
prefixed with material again.

Fixes #702

Add support for the appbar behavior described in the "Flexible space with image" section of https://www.google.com/design/spec/patterns/scrolling-techniques.html#scrolling-techniques-scrolling.

Introduces a new Tooltip class.
Adds support for tooltips to IconButton and Scaffold.
Adds some tooltips to various demos.

Also some tweaks to stack.dart that I made before I decided not to go
down a "CustomPositioned" route.

Now the splash goes to the edge. Also, fold popup_menu_item.dart into
popup_menu.dart for simplicity.

Fixes #266

We now draw radial reactions using Material.

Fixes #219

The slider in this patch works, but it's missing animations and many subtle
touches.

You can't actually pick a time with the time picker yet, but this patch adds
most of the plumbing.

http://www.google.com/design/spec/components/chips.html#chips-behavior

This patch adds support for deletable and non-deleteable chips, but doesn't yet
add support for contact chips. Also, demo the chips in a new Material Gallery
app that will let us demo our gallery of widgets in a single app.

PageRoute is now MaterialPageRoute.

This also changes the following:

- Now the HeroController is a Navigator observer, rather than a feature
  of HeroPageRoutes, which are gone. This means heroes can work between
  any kind of ModalRoute now.

- ModalPageRoute is moved from modal_barrier.dart to routes.dart.

- It allows routes to opt-out of their modal barrier being a shortcut to
  popping the route.

- Features of PageRoute that aren't Material-specific get promoted to
  ModalRoute features: storage, the subtree key, offstageness...

The AnimatedModalBarrier is still a ModalRoute feature.

The one we actually use is in material.dart.

Adds a dropdown menu button as shown in the Material design spec here: https://www.google.com/design/spec/components/buttons.html#buttons-dropdown-buttons. It's the video at the bottom of this section of the Material design spec.

This version of the component doesn't deal with scrollable menus or
constrain the height of the menu.

A MaterialList understands the sizing, padding, and scrollbar features of
Material Design lists.

Also, add CircleAvatar for showing the circular avatars that are commonly used
in material lists.

Enabled displaying a scrollbar in ScrollingLists. The scrollbar is painted as an "overlay", i.e. it's painted on top of the scrolling list's visible children.

Added an abstract Painter base class that encapsulates a paint method and the renderer that it's attached to. RenderBlockViewport and HomogenousViewport now support an overlayPainter property. If specified, RenderBlockViewport attaches itself to the overlayPainter when it's attached to the rendering tree. RenderBlockViewport now calls overlayPainter.paint() after it has painted its children.

Added an abstract ScrollingListPainter class that exposes ScrollingList's state which might be needed for painting. Like its scroll direction and scrollOffset. The ScrollingListPainter is notified when a scroll starts and ends.

Defined a Material-specific ScrollingListPainter that renders a scrollbar. The scrollbar thumb is faded in/out when the scroll starts/ends.

Added onScrollStart and onScrollEnd listeners to Scrollable.

MaterialApp assumes that you're using material design.

Also move radial reaction and fix imports for stats box.

These are now part of material.dart.

I've also removed the top-level description of the Sky package. Instead, we
should host that content on flutter.io.

Also, introduce Colors and Typography to hold the material colors and the
typography declarations. Previously we expected clients of these libraries to
import them into a namespace, but that doesn't play nice with re-exporting them
from material.dart.