Array.slice doesn't mutate original array, so it's fine with read only data.
Plus nuke unnecessary cast.

Review URL: http://codereview.chromium.org/5972004

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@6113 ce2b1a6d-e550-0410-aec6-3dcde31c8c00