anchor.diff:      root = _remove(root, key);
     }
   
  -  _TreeNode _remove(_TreeNode node, T searchKey) {
  +  TreeNode _remove(TreeNode node, T searchKey) {
       if (node == null) {
         return null;
       }
  
positive.code:     }
    }
  
    _insertNode(_TreeNode node, _TreeNode newNode) {
      if (newNode.key < node.key) {
        if (node.left == null) {
          node.left = newNode;
  
negative.code:   }
  }
  
  class TreeNode<T> {
    T key;
    TreeNode left;
    TreeNode right;
  
    TreeNode(this.key);
  }
  