anchor.diff:          for (var component in complex1.components) {
             if (component is CompoundSelector) {
               for (var simple in component.components) {
  -              var source = _sources[simple];
  -              if (source == null) continue;
  -              maxSpecificity = math.max(maxSpecificity, source.maxSpecificity);
  +              maxSpecificity =
  +                  math.max(maxSpecificity, _sourceSpecificity[simple] ?? 0);
               }
             }
           }
  
positive.code:       for (var component in complex.components) {
          if (component is CompoundSelector) {
            for (var simple in component.components) {
              _sources[simple] = complex;
            }
          }
        }
  
negative.code:   /// extensions.
    final _extensions = <SimpleSelector, Set<ExtendSource>>{};
  
    /// An expando from [SimpleSelector]s to integers.
    ///
    /// This tracks the maximum specificity of the [ComplexSelector] that
    /// originally contained each [SimpleSelector]. This allows us to ensure that
    /// we don't trim any selectors that need to exist to satisfy the [second law
    /// of extend][].
    ///
    /// [second law of extend]: https://github.com/sass/sass/issues/324#issuecomment-4607184
    final _sourceSpecificity = new Expando<int>();
  
    /// Extends [selector] with [source] extender and [target] the extendee.
    ///
  