* class.c (rb_special_singleton_class_of): utility function.
* vm_eval.c (eval_under): special deal for class variable scope with
  instance_eval.
* vm_eval.c (rb_obj_instance_eval, rb_obj_instance_exec): allow method
  definition in instance_eval of special constants.  [ruby-core:28324]
  [Bug #2788]


git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@36647 b2dd03c8-39d4-4d8f-98ff-823fe69b080e