Enable ES6 generators

R=rossberg@chromium.org

BUG=v8:2355
LOG=Y

Review URL: https://codereview.chromium.org/573963003

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@23974 ce2b1a6d-e550-0410-aec6-3dcde31c8c00