Fix small mistake in d8.js.

R=mvstanton@chromium.org
BUG=v8:2756

Review URL: https://codereview.chromium.org/18572009

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@15463 ce2b1a6d-e550-0410-aec6-3dcde31c8c00