anchor.diff:  
     final _extensions = <SimpleSelector, Set<SelectorList>>{};
   
  -  Set<PseudoSelector> _seen;
  +  final _sources = new Expando<ComplexSelector>();
   
     CssStyleRule addSelector(SelectorList selector, {FileSpan span}) {
       if (_extensions.isNotEmpty) selector = _extendList(selector);
  
positive.code:     for (var i = 0; i < lists.length; i++) {
        for (var complex1 in lists[i]) {
          // The maximum specificity of the sources that caused [complex1] to be
          // generated. In order for [complex1] to be removed, there must be another
          // selector that's a superselector of it *and* that has specificity
          // greater or equal to this.
          var maxSpecificity =
              maxBy(_sources(complex1), (source) => source.maxSpecificity) ?? 0;
  
          // Look in [result] rather than [lists] for selectors before [i]. This
          // ensures that we aren't comparing against a selector that's already
  
negative.code:       for (var component in complex.components) {
          if (component is CompoundSelector) {
            for (var simple in component.components) {
              _sources[simple] = complex;
              _selectors.putIfAbsent(simple, () => new Set()).add(rule);
            }
          }
  