anchor.diff:    /// A set of [ComplexSelector]s that were originally part of
     /// their component [SelectorList]s, as opposed to being added by `@extend`.
     ///
  -  /// This allows us to ensure that we do'nt trim any selectors that need to
  +  /// This allows us to ensure that we don't trim any selectors that need to
     /// exist to satisfy the [first law of extend][].
     ///
     /// [first law of extend]: https://github.com/sass/sass/issues/324#issuecomment-4607184
  
positive.code:     for (var i = lists.length - 1; i >= 0; i--) {
        middle:
        for (var complex1 in lists[i].reversed) {
          if (_originals.contains(complex1)) {
            // Make sure we don't include duplicate originals, which could happen
            // if a style rule extends a component of its own selector.
            for (var j = 0; j < numOriginals; j++) {
  
negative.code:   // itself. A selector is only removed if it's redundant with a selector in
    // another list. "Redundant" here means that one selector is a superselector
    // of the other. The more specific selector is removed.
    //
    // The [isOriginal] callback indicates which selectors are original to the
    // document, and thus should never be trimmed.
    List<ComplexSelector> _trim(List<List<ComplexSelector>> lists,
        bool isOriginal(ComplexSelector complex)) {
      // Avoid truly horrific quadratic behavior.
      //
      // TODO(nweiz): I think there may be a way to get perfect trimming without
  