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

@abarth @hixie

The TabBar's selection is now represented by a TabBarSelection object which encapsulates both the previous and currently selected indices and the Performance used to animate the selection indicator.

Added a TabBarView class which displays a tab's contents. It uses a shared TabBarSelection to stay in sync with a TabBar. The TabBarView scrolls in sync with the TabBar when the selection changes. Eventually it will allow one to fling the selection forward or backwards.

Added a tabBar property to ToolBar. Typically the corresponding TabBarView will be the body of the toolbar's Scaffold.

Removed TabNavigatorView and TabNavigator.

Added a widget gallery tabs demo page. Removed the old tabs demo.

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.