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 < compound.components.length; i++) {
        var simple = compound.components[i];
  
        // TODO: handle extending into pseudo selectors, tracking sources, extend
        // failures
  
        var extenders = _extensions[simple];
        if (extenders == null) continue;
  
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);
            }
          }
  