anchor.diff:        BidirectionalMap<FunctionExpression, MemberDeclaration<FunctionRule>>();
     final _globalDeclarations = <MemberDeclaration>{};
     final _sources = <SassNode, ReferenceSource>{};
  +  final _importers = <Uri, Importer>{};
   
     /// The current global scope.
     ///
  
positive.code:   /// functions referenced within `get-function` calls (those nodes instead
    /// map to the [ReferenceSource] for the `sass:meta` module).
    final Map<SassNode, ReferenceSource> sources;
  
    /// An iterable of all member declarations.
    Iterable<MemberDeclaration> get allDeclarations =>
  
negative.code:         getFunctionReferences =
              UnmodifiableBidirectionalMapView(getFunctionReferences),
          globalDeclarations = UnmodifiableSetView(globalDeclarations),
          sources = UnmodifiableMapView(sources),
          importers = UnmodifiableMapView(importers);
  
    /// Constructs a new [References] object based on a [stylesheet] (imported by
    /// [importer]) and its dependencies.
  