Check that constructor is a FunctionMirror before calling .name(), otherwise we may end up calling name method on a mirror object that doesn't have it.

Review URL: http://codereview.chromium.org/271053

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@3057 ce2b1a6d-e550-0410-aec6-3dcde31c8c00