Rev the version of the flutter framework (#8257)

Now that all the breaking scrolling changes are in, this seems like a reasonble
time to rev package:flutter's version number.

packages/flutter/pubspec.yaml

 name: flutter
-version: 0.0.21
+version: 0.0.22
 author: Flutter Authors <flutter-dev@googlegroups.com>
 description: A framework for writing Flutter applications
 homepage: http://flutter.io