Patch from Mathias Bynens <mathias@qiwi.be>
BUG=1958
TEST=Try `var a\u200c\u200d;` and confirm it doesn't throw an error.

Review URL: https://chromiumcodereview.appspot.com/9433031
Patch from Mathias Bynens <mathias@qiwi.be>.

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@10800 ce2b1a6d-e550-0410-aec6-3dcde31c8c00