1. Take overrides into account.
2. Do get if a cache package is not installed.

BUG=http://dartbug.com/14590, http://dartbug.com/15180
R=nweiz@google.com

Review URL: https://codereview.chromium.org//83173011

git-svn-id: https://dart.googlecode.com/svn/branches/bleeding_edge@30832 260f80e4-7a28-3924-810f-c04153c831b5