If we can tell (using $include/$exclude) that a dependency's
transformer can't run on any public files in that package, don't
bother loading that transformer at all.

R=rnystrom@google.com
BUG=17534

Review URL: https://codereview.chromium.org//363543003

git-svn-id: https://dart.googlecode.com/svn/branches/bleeding_edge@37925 260f80e4-7a28-3924-810f-c04153c831b5