ES6 spec tacitly allows them, and they are allowed in Firefox and in
WebKit/Blink.

R=bmeurer@chromium.org,rossberg@chromium.org

Review URL: https://codereview.chromium.org/18769005

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@15579 ce2b1a6d-e550-0410-aec6-3dcde31c8c00