anchor.diff:  class SplayTreeMap<K, V> extends _SplayTree<K, _SplayTreeMapNode<K, V>>
       with MapMixin<K, V> {
     _SplayTreeMapNode<K, V>? _root;
  -  final _SplayTreeMapNode<K, V> _dummy = _DummySplayTreeMapNode<K, V>();
  +  final _SplayTreeMapNode<K, V> _dummy = _SplayTreeMapNode<K, V>(null, null);
   
     Comparator<K> _comparator;
     _Predicate _validKey;
  
positive.code:     // the key to the root of the tree.
      int comp = _splay(key);
      if (comp == 0) {
        _root!.value = value;
        return;
      }
      _addNewRoot(_SplayTreeMapNode(key, value), comp);
  
negative.code: class SplayTreeSet<E> extends _SplayTree<E, _SplayTreeNode<E>>
      with IterableMixin<E>, SetMixin<E> {
    _SplayTreeNode<E>? _root;
    final _SplayTreeNode<E> _dummy = _SplayTreeNode<E>(null);
  
    Comparator<E> _comparator;
    _Predicate _validKey;
  