anchor.diff:  
         // Walk through the superclasses, and record the types
         // implemented by that type on the superclasses.
  -      ClassEntity superclass = _elementMap.getSuperClass(cls);
  +      ClassEntity? superclass = _elementMap.getSuperClass(cls);
         while (superclass != null) {
           Set<ClassEntity> typesImplementedBySubclassesOfCls =
               typesImplementedBySubclasses.putIfAbsent(superclass, () => {});
  
positive.code:   @override
    void registerTypeInstantiation(
        InterfaceType type, ClassUsedCallback classUsed,
        {ConstructorEntity constructor}) {
      ClassEntity cls = type.element;
      InstantiationInfo info =
          _instantiationInfo.putIfAbsent(cls, () => InstantiationInfo());
  
negative.code:       return false;
      }
  
      ClassEntity? current = cls;
      while (current != null && processClass(current)) {
        current = _elementEnvironment.getSuperClass(current);
      }
    }
  
  