Apparently methodOverride callback isn't used anymore in this context. Forgot to remove.

lib/class.js

@@ -529,13 +529,6 @@ var extend = ( function( extending )
                     abstract_methods.__length--;
                 }
             },
-
-            methodOverride: function( name, pre, func )
-            {
-                return util.overrideMethod(
-                    name, pre, func, abstract_methods
-                );
-            },
         } );
 
         // reference to the parent prototype (for more experienced users)