Resolve libraries against their canonical URI.

This is fixes a regression from r10895.

BUG=https://code.google.com/p/dart/issues/detail?id=21864
R=johnniwinther@google.com

Review URL: https://codereview.chromium.org//803543002

git-svn-id: https://dart.googlecode.com/svn/branches/bleeding_edge@42359 260f80e4-7a28-3924-810f-c04153c831b5