This avoids using the Sparse methods on objects with non-enumerable elements,
which can cause the 'enumerable: false' bit to get lost in the operation.

R=mstarzinger@chromium.org

Review URL: https://codereview.chromium.org/672323003

Cr-Commit-Position: refs/heads/master@{#24883}
git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@24883 ce2b1a6d-e550-0410-aec6-3dcde31c8c00