Previously, two transformers from the same library but with different
configuration was considered completely different from one another. This was not
only inefficient but incorrect: it meant that a transformer could be loaded with
the wrong code.

R=rnystrom@google.com
BUG=19261

Review URL: https://codereview.chromium.org//344673002

git-svn-id: https://dart.googlecode.com/svn/branches/bleeding_edge@37511 260f80e4-7a28-3924-810f-c04153c831b5