anchor.diff:    /// [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}) {
  +  YamlScalar.wrap(this.value, {sourceUrl, this.style = ScalarStyle.ANY})
  +      : super._(NullSpan(sourceUrl)) {
       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;
  -  }
  +  YamlScalar.internal(this.value, ScalarEvent scalar)
  +      : style = scalar.style,
  +        super._(scalar.span);
   
     /// Users of the library should not use this constructor.
     YamlScalar.internalWithSpan(this.value, SourceSpan span)
  -      : style = ScalarStyle.ANY {
  -    _span = span;
  -  }
  +      : style = ScalarStyle.ANY,
  +        super._(span);
   
     @override
     String toString() => value.toString();
  
positive.code:   /// [SourceSpan.message] can be used to produce a human-friendly message about
    /// this node.
    SourceSpan get span => _span;
  
    late SourceSpan _span;
  
    /// The inner value of this node.
    ///
  
negative.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),
          super._(span);
  
    @override
    dynamic operator [](key) => nodes[key]?.value;
  