only be shadowed by eval-introduced variables we usually do not need
to do a context lookup in the runtime system.

Review URL: http://codereview.chromium.org/2672001

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@4803 ce2b1a6d-e550-0410-aec6-3dcde31c8c00