* Add a map of relative to absolute paths for assets located outside the app's
source directory
* If a "packages" directory exists, obtain assets from there instead of
using the packages/ prefix to indicate package map lookup
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#2313Fixes#2218Fixes#2009Fixes#994
