Since we are using private symbols for the internal slots we need to
check for a local property.

BUG=None
LOG=Y
R=rossberg@chromium.org

Review URL: https://codereview.chromium.org/268363011

Patch from Erik Arvidsson <arv@chromium.org>.

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@21399 ce2b1a6d-e550-0410-aec6-3dcde31c8c00