A potential issue with this change is creating lots of maps when
objects flip between fast/slow elements modes.  We could add special
transitions to avoid this.  Yet testing this on our benchmarks, gmail,
and wave seems to indicate that this is not a real problem.

Review URL: http://codereview.chromium.org/2870018

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@4941 ce2b1a6d-e550-0410-aec6-3dcde31c8c00