Create a "PackageRef" class that defines a versionless package reference.

R=nweiz@google.com

Review URL: https://codereview.chromium.org//13952013

git-svn-id: https://dart.googlecode.com/svn/branches/bleeding_edge/dart@22187 260f80e4-7a28-3924-810f-c04153c831b5
8 files changed