anchor.diff:    final Duration minDelay;
     final Duration maxDelay;
   
  -  VoidCallback _closure;
  -
  -  Timer _minTimer;
  -  Timer _maxTimer;
  +  VoidCallback? _closure;
  +
  +  Timer? _minTimer;
  +  Timer? _maxTimer;
   
     void invoke(VoidCallback closure) {
       _closure = closure;
  
positive.code:   }
  
    @override
    void fillRange(int start, int end, [T fill]) {
      throw Exception('Cannot modify the content of ImmutableList');
    }
  
  
negative.code:   }
  
    @override
    bool remove(Object? element) {
      throw Exception('Cannot modify the content of ImmutableList');
    }
  
  