For-of on null or undefined is an error

The latest ES6 draft changed the behavior of for-of on null / undefined,
which for once is a simplification.

R=rossberg@chromium.org
BUG=

Review URL: https://codereview.chromium.org/416033002

Patch from Andy Wingo <wingo@igalia.com>.

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@22602 ce2b1a6d-e550-0410-aec6-3dcde31c8c00