probe code object, and really raise an exception if SplayTree.Remove
is called with a key that cannot be found. 
Review URL: http://codereview.chromium.org/42597

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@1606 ce2b1a6d-e550-0410-aec6-3dcde31c8c00