anchor.diff:    /// Calls [f] for each non-null entity.
     void forEach<E0 extends E>(void f(E0 entity)) {
       for (int index = 0; index < _list.length; index++) {
  -      E entity = _list[index];
  +      final entity = _list[index] as E0?;
         if (entity != null) {
           f(entity);
         }
  
positive.code:   void _preRegister<E0 extends E, V0 extends V>(E0 entity, V0 env) {
      assert(
          !_closed, "Trying to register $entity @ ${_list.length} when closed.");
      assert(entity != null);
      assert(entity._index == null);
      assert(
          _list.length == _env.length,
          'Env list length ${_env.length} inconsistent '
  
negative.code:   void postRegisterData<E0 extends E, D0 extends D>(E0 entity, D0 data) {
      assert(
          !_closed, "Trying to register $entity @ ${_list.length} when closed.");
      assert((entity as dynamic) != null); // TODO(48820): Remove.
      assert(
          (_list.length - 1) == _data.length,
          'Data list length ${_data.length} inconsistent '
  