It's a port of http://code.google.com/p/v8/source/detail?r=3209 to x64 platform.
That allows invocation of callbacks without going into runtime.

Review URL: http://codereview.chromium.org/2801008

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@5141 ce2b1a6d-e550-0410-aec6-3dcde31c8c00