This is prep work for global packages, which need to ensure their
dependencies are local, but do not want to create symlinks for them.

R=nweiz@google.com

Review URL: https://codereview.chromium.org//357483004

git-svn-id: https://dart.googlecode.com/svn/branches/bleeding_edge@37672 260f80e4-7a28-3924-810f-c04153c831b5