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This allows assigning directly as `list.last = value` instead of having to use the longer `list[list.length - 1] = value`. It's just an abbreviation, like `list.last` is for reading. If you use a List as a stack (which is a perfectly good use of it), modifying the last element like this is a natural operation. R=sgjesse@google.com Review URL: https://codereview.chromium.org//315173005 git-svn-id: https://dart.googlecode.com/svn/branches/bleeding_edge/dart@37255 260f80e4-7a28-3924-810f-c04153c831b5