Add back the duplicate property checker

We're not quite ready to make this change.

BUG=v8:3498
LOG=Y
R=rossberg@chromium.org

Review URL: https://codereview.chromium.org/491053002

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@23284 ce2b1a6d-e550-0410-aec6-3dcde31c8c00
Merge remote-tracking branch 'upstream/master'