anchor.diff:        BidirectionalMap<FunctionExpression, MemberDeclaration<FunctionRule>>();
     final _globalDeclarations = <MemberDeclaration>{};
     final _sources = <SassNode, ReferenceSource>{};
  -  final _importers = <Uri, Importer>{};
   
     /// The current global scope.
     ///
  
positive.code:   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;
  
negative.code:               (source is CurrentSource || source is ForwardSource)) {
              _declarationSources[declaration] = importSource;
            }
            _importer = oldImporter;
          }
        }
      }
    }
  