When calling Object.isExtensible we did not do a check for the global
js proxy. This caused the check on the extensible bit on the map to
return true, even when the bit was set to false on the global js
object.


Review URL: http://codereview.chromium.org/6450003

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@6677 ce2b1a6d-e550-0410-aec6-3dcde31c8c00