behavior.  Only force the introduction of a variable directly on the
global object if there is an explicit initial value in a variable
declaration.

BUG=http://crbug.com/12548
Review URL: http://codereview.chromium.org/151191

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@2326 ce2b1a6d-e550-0410-aec6-3dcde31c8c00