anchor.diff:  
     void _cancel(int id) {
       // TODO: sorted map by sequence
  -    var cb = _callbacks.firstWhere((t) => t.sequence == id);
  -    cb._timer.cancel();
  +    var cb = _callbacks.firstWhere((cb) => cb.sequence == id);
  +    cb._timer?.cancel();
       _callbacks.remove(cb);
       // TODO: also remove timer associated with id
     }
   
  -  bool _isActive(int id) => _callbacks.any((t) => t.sequence == id);
  +  bool _isActive(int id) => _callbacks.any((cb) => cb.sequence == id);
   
     int _ticks(int id) {
       throw UnimplementedError();
  
positive.code:     if (_pausedAt != null) return;
      _pausedAt = _parentOffset();
  
      while (_timers.isNotEmpty) {
        _timers.removeFirst().cancel();
      }
    }
  
    void resume() {
  
negative.code:       var remaining = cb.nextCall - offset;
  
        var t = _zone.parent.createTimer(remaining, () {
          var next = _popCallback();
  
          if (next.isPeriodic) {
            var pt = _zone.parent.createPeriodicTimer(cb.duration, (_) {
              var next = _popCallback().next();
              _callbacks.add(next);
              next.callback(_CallbackTimer(next.sequence, this));
            });
            next._timer = pt;
            _callbacks.remove(next);
            _callbacks.add(next.next());
          }
  
          next.callback();
        });
  
        cb._timer = t;
      }
    }
  