anchor.diff:      final List<_HeroQuestState> _newHeroes = <_HeroQuestState>[];
       for (_HeroMatch heroPair in heroes.values) {
         assert(heroPair.from != null || heroPair.to != null);
  +      if ((heroPair.from == null && !heroPair.to.alwaysAnimate) ||
  +          (heroPair.to == null && !heroPair.from.alwaysAnimate))
  +        continue;
         _HeroManifest from = heroPair.from?._takeChild(animationArea);
         assert(heroPair.to == null || heroPair.to is HeroState);
         _HeroManifest to = heroPair.to?._takeChild(animationArea);
  
positive.code: 
    _HeroMode _mode = _HeroMode.constructing;
    Size _size;
  
    _HeroManifest _takeChild(Rect animationArea) {
      assert(_mode == _HeroMode.measured || _mode == _HeroMode.taken);
  
negative.code:     Key key,
      this.tag,
      this.child,
      this.turns: 1,
      this.alwaysAnimate: false
    }) : super(key: key) {
      assert(tag != null);
    }
  