r8602: Fix a potential crash in const declaration.

Declaration of const lookup slots would trigger an assertion if there was a
setter somewhere in the prototype chain, and that setter was shadowed by a
non-readonly data property also in the prototype chain.

R=ager@chromium.org

Review URL: http://codereview.chromium.org/7335009

git-svn-id: http://v8.googlecode.com/svn/branches/3.2@8606 ce2b1a6d-e550-0410-aec6-3dcde31c8c00