Do not do standard property lookup on hidden properties object as it might
reach Object.prototype which can be altered to forge identity hashes.
Instead do only local lookup.

Review URL: http://codereview.chromium.org/6472001

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@6728 ce2b1a6d-e550-0410-aec6-3dcde31c8c00