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It is possible that we may be stepping into an optimized frame otherwise. In fact, the current frame may be optimized if the isolate was interrupted instead of hitting a breakpoint. BUG= R=hausner@google.com Review URL: https://codereview.chromium.org//797343002 git-svn-id: https://dart.googlecode.com/svn/branches/bleeding_edge/dart@42348 260f80e4-7a28-3924-810f-c04153c831b5