anchor.diff:  /// corresponding data object of type [D].
   abstract class EntityDataMapBase<E extends _Indexed, D>
       extends EntityMapBase<E> {
  -  final List<D> _data = <D>[];
  +  final List<D?> _data = <D?>[];
   
     /// Returns the data object stored for the [index]th entity.
     D getData(E entity) {
  
positive.code:       throw StateError('Data is in the process of being created.');
      }
      for (int index = 0; index < _list.length; index++) {
        E entity = _list[index];
        if (entity != null) {
          f(entity, _data[index]);
        }
      }
    }
  
negative.code:       E0 entity, D0 data, V0 env) {
      assert(
          !_closed, "Trying to register $entity @ ${_list.length} when closed.");
      assert((entity as dynamic) != null); // TODO(48820): Remove.
      assert(
          _list.length == _data.length,
          'Data list length ${_data.length} inconsistent '
  