Importing the public libraries caused a name conflict with dart:sky because we
assume people will import dart:sky into a namespace, so I've also changed
skysprites to import dart:sky into a namespace.

Using ImageResource solves two problems:

1) Listeners can be notified synchronously when the sky.Image is already
   available. This change removes flash of 0x0 layout when moving an
   already-cached image around in the render tree.

2) In the future, when we support animated images, we can notify listeners
   multiple times whenever a new image is available.