Properly delete cached dependencies that should no longer be cached.

This fixes the "Could not find asset" issue people were seeing on "pub
upgrade".

R=rnystrom@google.com
BUG=21810

Review URL: https://codereview.chromium.org//792993004

git-svn-id: https://dart.googlecode.com/svn/branches/bleeding_edge@42273 260f80e4-7a28-3924-810f-c04153c831b5