Merge pull request #641 from Hixie/dependencies

Update READMEs for painting/ and animation/ to be up to date with current dependencies.

packages/flutter/lib/animation/README.md

 This directory contains animation-related libraries that only depend
-on core Dart libraries and ../base/*.
+on core Dart libraries, the Newton Dart library, and ../base/*.

packages/flutter/lib/painting/README.md

+This directory contains painting-related libraries that only depend on
+core Dart libraries, ../base/*, and ../animation/*. Note that
+../animation/* depends on the Newton Dart library also.