anchor.diff:  
     Iterable<ConstantUse> get constantUses => const [];
   
  -  void _forEach<U>(Iterable<U> uses, void Function(MemberEntity, U) visitUse) =>
  +  void _forEach<U>(
  +          Iterable<U> uses, void Function(MemberEntity?, U) visitUse) =>
         uses.forEach((use) => visitUse(member, use));
   
  -  void forEachDynamicUse(void Function(MemberEntity, DynamicUse) visitUse) =>
  +  void forEachDynamicUse(void Function(MemberEntity?, DynamicUse) visitUse) =>
         _forEach(dynamicUses, visitUse);
  -  void forEachStaticUse(void Function(MemberEntity, StaticUse) visitUse) =>
  +  void forEachStaticUse(void Function(MemberEntity?, StaticUse) visitUse) =>
         _forEach(staticUses, visitUse);
  -  void forEachTypeUse(void Function(MemberEntity, TypeUse) visitUse) =>
  +  void forEachTypeUse(void Function(MemberEntity?, TypeUse) visitUse) =>
         _forEach(typeUses, visitUse);
  -  void forEachConstantUse(void Function(MemberEntity, ConstantUse) visitUse) =>
  +  void forEachConstantUse(void Function(MemberEntity?, ConstantUse) visitUse) =>
         _forEach(constantUses, visitUse);
   
     bool get isEmpty => true;
  
positive.code: class WorldImpact {
    const WorldImpact();
  
    MemberEntity get member => null;
  
    Iterable<DynamicUse> get dynamicUses => const [];
  
  
negative.code: 
    @override
    void registerConstantUse(ConstantUse constantUse) {
      assert((constantUse as dynamic) != null); // TODO(48820): Remove when sound.
      (_constantUses ??= Setlet()).add(constantUse);
    }
  
    @override
  