anchor.diff:  import 'selection_model.dart';
   
   typedef bool IsIndeterminate(var entity);
  +
  +/// Count getter to return when there is no valid count
  +final CountGetter _noCountGetter = () => -1;
   
   /// Selection model interface that is meant to be used with the table view
   /// component.
  
positive.code:       _totalEntitiesCountController.stream;
  
    @override
    CountGetter entitiesOnPageCount = () => -1;
  
    @override
    bool supportsMultiplePageSelection = true;
  
    @override
    SelectableGetter<T> getSelectable;
  
    /// Returns number of items selected in the model.
    int get _selectedCount => _backingModel.selectedValues.length;
  
negative.code: 
    @override
    bool get isPageSelected =>
        supportsSelectAll &&
        selectedCount > 0 &&
        selectedCount == entitiesOnPageCount();
  
    @override
    void onRemove(T entity) {
  