Fix issue 191:

Fix issue 191:

    http://code.google.com/p/v8/issues/detail?id=191

Accessor setters should not be called for eval-introduced context
extension objects.

This change fixes the issue, but I think it is time to separate out
the lookup routines for eval-introduced context extension objects.
There is no reason to use the general lookup routines for this.  I'll
do that in a separate changelist.
Review URL: http://codereview.chromium.org/18090

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@1076 ce2b1a6d-e550-0410-aec6-3dcde31c8c00