This uses a global sentinel as a replacement for undefined keys, which
are not supported internally but required for Harmony sets and maps.

R=rossberg@chromium.org
BUG=v8:1622
TEST=mjsunit/harmony/collections

Review URL: http://codereview.chromium.org/8439069

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@9873 ce2b1a6d-e550-0410-aec6-3dcde31c8c00