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:     return _search(root, key);
    }
  
    bool _search(_TreeNode node, T searchKey) {
      if (node == null) {
        return false;
      }
  
negative.code: class BinarySearchTree<T extends Comparable> {
    TreeNode<T> root;
  
    insert(T key) {
      var newNode = new TreeNode<T>(key);
  
      if (root == null) {
        root = newNode;
  