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

Now you can colorize an Icon simply by proving a Color for the Icon.

Previously we used a positional argument for widgets that had multiple
children. Now we use a named argument that defaults to an empty list.

Fixes #241

After this patch, if you try to use a widget that depends on being enclosed in
a material, you now get an assert and a debugPrint if you're not inside a
material.

Fixes #243

Fixes #464

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.