anchor.diff:    References build(Stylesheet stylesheet, Importer importer) {
       _importer = importer;
       _scope = Scope();
  +    _importers[stylesheet.span.sourceUrl] = importer;
       _moduleScopes[stylesheet.span.sourceUrl] = _scope;
       _declarationSources = {};
       _moduleSources[stylesheet.span.sourceUrl] = _declarationSources;
  
positive.code:     if (result == null) return null;
      var stylesheet = result.item2;
      var canonicalUrl = stylesheet.span.sourceUrl;
      if (!_moduleScopes.containsKey(canonicalUrl)) {
        var oldScope = _scope;
        _scope = Scope();
        _moduleScopes[canonicalUrl] = _scope;
        var oldSources = _declarationSources;
        _declarationSources = {};
        _moduleSources[canonicalUrl] = _declarationSources;
        var oldImporter = _importer;
        _importer = result.item1;
        visitStylesheet(stylesheet);
        _checkUnresolvedReferences(_scope);
        _importer = oldImporter;
        _scope = oldScope;
        _declarationSources = oldSources;
      }
      return canonicalUrl;
    }
  
  
negative.code:       BidirectionalMap<FunctionExpression, MemberDeclaration<FunctionRule>>();
    final _globalDeclarations = <MemberDeclaration>{};
    final _sources = <SassNode, ReferenceSource>{};
    final _importers = <Uri, Importer>{};
  
    /// The current global scope.
    ///
  