anchor.diff:    /// [SourceSpan.message] can be used to produce a human-friendly message about
     /// this node.
     SourceSpan get span => _span;
  +  SourceSpan _span;
   
  -  late SourceSpan _span;
  +  YamlNode._(this._span);
   
     /// The inner value of this node.
     ///
  
positive.code:   /// [sourceUrl] is passed, it's used as the [SourceSpan.sourceUrl].
    ///
    /// [sourceUrl] may be either a [String], a [Uri], or `null`.
    YamlScalar.wrap(this.value, {sourceUrl, this.style = ScalarStyle.ANY}) {
      ArgumentError.checkNotNull(style, 'style');
      _span = NullSpan(sourceUrl);
    }
  
    /// Users of the library should not use this constructor.
    YamlScalar.internal(this.value, ScalarEvent scalar) : style = scalar.style {
      _span = scalar.span;
    }
  
    /// Users of the library should not use this constructor.
    YamlScalar.internalWithSpan(this.value, SourceSpan span)
        : style = ScalarStyle.ANY {
      _span = span;
    }
  
    @override
    String toString() => value.toString();
  
negative.code: 
    /// Users of the library should not use this constructor.
    YamlList.internal(List<YamlNode> nodes, SourceSpan span, this.style)
        : nodes = UnmodifiableListView<YamlNode>(nodes),
          super._(span);
  
    @override
    dynamic operator [](int index) => nodes[index].value;
  