This shaves off a pretty significant amount of time for packages that are
transformed but whose transformers never change (I saw a ~450ms gain in one of
my tests).

R=rnystrom@google.com
BUG=20859

Review URL: https://codereview.chromium.org//559833004

git-svn-id: https://dart.googlecode.com/svn/branches/bleeding_edge@40392 260f80e4-7a28-3924-810f-c04153c831b5