Unify and fix checkers for duplicate object literal properties.

R=ulan@chromium.org
TEST=preparser/duplicate-property

Review URL: https://codereview.chromium.org/25755002

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@17114 ce2b1a6d-e550-0410-aec6-3dcde31c8c00