Class.isAbstract() now defined using define_secure_prop

lib/class.js

@@ -321,10 +321,10 @@ function attach_abstract( func, methods )
      *
      * @return  {Boolean}  true if class is abstract, otherwise false
      */
-    func.isAbstract = function()
+    define_secure_prop( func, 'isAbstract', function()
     {
         return is_abstract;
-    };
+    });
 
     // attach the list of abstract methods to the class (make the copy of the
     // methods to ensure that they won't be gc'd or later modified and screw up