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We need to exclude a common part of a prefix/suffix. Also, I decided not to improve diff calculation, at least in the short term. So, I'm removing diff.dart and the TODO comment. R=brianwilkerson@google.com BUG= Review URL: https://codereview.chromium.org//626883003 git-svn-id: https://dart.googlecode.com/svn/branches/bleeding_edge/dart@40903 260f80e4-7a28-3924-810f-c04153c831b5