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When a [PhaseOutput.removeListeners] was called, the output was re-added to the [Phase] but its removal event wasn't re-subscribed. This meant that if that output was later removed, the phase got into an inconsistent state where it would perform operations on a removed output. R=rnystrom@google.com BUG= Review URL: https://codereview.chromium.org//23803006 git-svn-id: https://dart.googlecode.com/svn/branches/bleeding_edge/dart@27170 260f80e4-7a28-3924-810f-c04153c831b5