Clean up global_packages.dart.

BUG=http://dartbug.com/19962
R=rnystrom@google.com

Review URL: https://codereview.chromium.org//489823002

git-svn-id: https://dart.googlecode.com/svn/branches/bleeding_edge@39466 260f80e4-7a28-3924-810f-c04153c831b5