anchor.diff:  
     @override
     bool get isPageSelected =>
  -      selectedCount > 0 && selectedCount == entitiesOnPageCount();
  +      supportsSelectAll &&
  +      selectedCount > 0 &&
  +      selectedCount == entitiesOnPageCount();
   
     @override
     void onRemove(T entity) {
  
positive.code: 
    bool _allSelected = false;
  
    CountGetter _totalEntitiesCount = () => -1;
  
    @override
    CountGetter get totalEntitiesCount => _totalEntitiesCount;
  
    @override
    set totalEntitiesCount(CountGetter value) {
  
negative.code:       _totalEntitiesCountController.stream;
  
    @override
    CountGetter entitiesOnPageCount = _noCountGetter;
  
    @override
    bool supportsMultiplePageSelection = true;
  
    @override
    SelectableGetter<T> getSelectable;
  
    @override
    bool supportsSelectAll = true;
  
    /// Returns number of items selected in the model.
    int get _selectedCount => _backingModel.selectedValues.length;
  