gets converted fo "foo,bar,baz".split(",").  If the inputs are
symbols we cache the result and make the substrings into symbols.
Review URL: http://codereview.chromium.org/7782025

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@9164 ce2b1a6d-e550-0410-aec6-3dcde31c8c00