anchor.diff:    @override
     void registerStaticUse(StaticUse staticUse) {
       _staticUses ??= Setlet.of(worldImpact.staticUses);
  -    _staticUses.add(staticUse);
  +    _staticUses!.add(staticUse);
     }
   
     @override
  
positive.code: 
    @override
    void registerTypeUse(TypeUse typeUse) {
      assert(typeUse != null);
      _typeUses ??= Setlet();
      _typeUses.add(typeUse);
    }
  
    @override
  
negative.code: class WorldImpactBuilderImpl extends WorldImpactBuilder {
    /// The [MemberEntity] associated with this set of impacts. Maybe null.
    @override
    final MemberEntity? member;
  
    // TODO(johnniwinther): Do we benefit from lazy initialization of the
    // [Setlet]s?
    Set<DynamicUse>? _dynamicUses;
    Set<StaticUse>? _staticUses;
    Set<TypeUse>? _typeUses;
    Set<ConstantUse>? _constantUses;
  
    WorldImpactBuilderImpl([this.member]);
  
  