And fix the zillion issues that uncovered.

This reverts commit f41b3411, reversing
changes made to e33d8d96.

This was a bad check-in due to my mangling uploading a new version of the branch from a different machine.

This reverts https://github.com/flutter/flutter/pull/2639 and will be replaced by https://github.com/flutter/flutter/pull/2640

And fix the zillion issues that uncovered.

This makes skyanalyzer also check the examples, and fixes everything it
found there.

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.