anchor.diff:     * The returned list contains all items that were selected.
      */
     List<TreeItem> selectWhere(bool test(TreeItem item)) {
  -    // Deselect all items.
  -    selectedItems.forEach((el) => el.selected = false);
  +    clearSelection();
       
       // Find the items in the tree.
       List<TreeItem> foundItems = findItems(test);
  
positive.code:   }
    
    /**
     * Selects all items of the tree that satisfy the predicate [test]. All 
     * previously selected items are deselected. The tree is expanded so that all 
     * selected items are shown. No events are fired.
  
negative.code:       item.selected = true;
        ensureVisible(item);
      });
      
      return foundItems;
    }
    
    /**
  