anchor.diff:      }
     }
   
  -  _TreeNode _findMinNode(_TreeNode node) {
  +  TreeNode _findMinNode(TreeNode node) {
       while (node != null && node.left != null) {
         node = node.left;
       }
  
positive.code:     root = _remove(root, key);
    }
  
    _TreeNode _remove(_TreeNode node, T searchKey) {
      if (node == null) {
        return null;
      }
  
negative.code:     _preOrderTraverse(root, callback);
    }
  
    _preOrderTraverse(TreeNode node, void callback(T key)) {
      if (node != null) {
        if (callback !=null) {
          callback(node.key);
  