Source slot might belong to a dead old object and we might allocate a new object over it when evacuating a new space object this slot points to. In this case if we update slot after migrating object we will write into migrated object.

R=erik.corry@gmail.com

Review URL: http://codereview.chromium.org/8698022

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@10082 ce2b1a6d-e550-0410-aec6-3dcde31c8c00