global object (using hidden prototypes):

1) setters might be not on the global object itself, but on its prototypes;
2) if property on one of prototypes is readonly, we could shadow it.
Review URL: http://codereview.chromium.org/434035

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@3362 ce2b1a6d-e550-0410-aec6-3dcde31c8c00