anchor.diff:        BidirectionalMap<FunctionExpression, MemberDeclaration<FunctionRule>>();
     final _globalDeclarations = <MemberDeclaration>{};
     final _sources = <SassNode, ReferenceSource>{};
  -  final _importers = <Uri, Importer>{};
   
     /// The current global scope.
     ///
  
positive.code:       BidirectionalMap<FunctionExpression, MemberDeclaration<FunctionRule>>
            getFunctionReferences,
        Set<MemberDeclaration> globalDeclarations,
        Map<SassNode, ReferenceSource> sources,
        Map<Uri, Importer> importers)
        : variables = UnmodifiableBidirectionalMapView(variables),
          variableReassignments =
              UnmodifiableBidirectionalMapView(variableReassignments),
  
negative.code:         getFunctionReferences =
              UnmodifiableBidirectionalMapView(getFunctionReferences),
          globalDeclarations = UnmodifiableSetView(globalDeclarations),
          sources = UnmodifiableMapView(sources);
  
    /// Constructs a new [References] object based on a [stylesheet] (imported by
    /// [importer]) and its dependencies.
  