Reland "Handlify concat string and substring."

This relands commit r17490.

BUG=
R=ulan@chromium.org

Review URL: https://codereview.chromium.org/114943004

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@18408 ce2b1a6d-e550-0410-aec6-3dcde31c8c00