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In addition to cleaning up Phase, this will make it easier to actually remove phases when transformers are updated. R=rnystrom@google.com BUG= Review URL: https://codereview.chromium.org//23363002 git-svn-id: https://dart.googlecode.com/svn/branches/bleeding_edge/dart@26469 260f80e4-7a28-3924-810f-c04153c831b5