Otherwise transition will hide CONSTANT_FUNCTION property with the same name residing on the prototype.

R=danno@chromium.org
BUG=v8:1883

Review URL: http://codereview.chromium.org/9064003

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@10312 ce2b1a6d-e550-0410-aec6-3dcde31c8c00