anchor.diff:  
     /// Users of the library should not use this constructor.
     YamlList.internal(List<YamlNode> nodes, SourceSpan span, this.style)
  -      : nodes = UnmodifiableListView<YamlNode>(nodes) {
  -    _span = span;
  -  }
  +      : nodes = UnmodifiableListView<YamlNode>(nodes),
  +        super._(span);
   
     @override
     dynamic operator [](int index) => nodes[index].value;
  
positive.code: 
    /// Users of the library should not use this constructor.
    YamlMap.internal(Map<dynamic, YamlNode> nodes, SourceSpan span, this.style)
        : nodes = UnmodifiableMapView<dynamic, YamlNode>(nodes) {
      _span = span;
    }
  
    @override
    dynamic operator [](key) => nodes[key]?.value;
  
negative.code:   /// [SourceSpan.message] can be used to produce a human-friendly message about
    /// this node.
    SourceSpan get span => _span;
    SourceSpan _span;
  
    YamlNode._(this._span);
  
    /// The inner value of this node.
    ///
  