Apparently this is no longer in /usr/include like it used to be on OS X.
This time, String::New no longer exists: v8/v8@e257225
Thanks to Travis Heppe for the implementation, and David Day for implementation comments.
On my OS X system with v8 version 188.8.131.52, this appears to work for all but the DataView class. As far as I can tell the modern version of v8 supports this class too.