anchor.diff:  ClassFields fieldsStack = ClassFields();
   
   Map<String, List<String>> drillIn(
  -  MemoryController controller,
  +  MemoryController? controller,
     AnalysisReference librariesNode,
     List<Reference> references, {
     createTreeNodes = false,
  
positive.code:   bool isClassMatched(String className) =>
        matcherDrillIn.containsKey(className);
  
    List<List<String>> _findClassMatch(String className) =>
        matcherDrillIn[className];
  
    // TODO(terry): Change to be less strict.  Look for subclass or parentage
  
negative.code:       result['externals'] = externalsToAnalyze;
      } else if (library.isFiltered) {
        final filtersToAnalyze = <ClassReference>[];
        for (final LibraryReference libraryRef
            in library.children as Iterable<LibraryReference>) {
          for (final ClassReference classRef
              in libraryRef.children as Iterable<ClassReference>) {
            final HeapGraphClassLive liveClass = classRef.actualClass!;
            if (_classMatcher(liveClass)) {
              filtersToAnalyze.add(classRef);
              final instances = liveClass.getInstances(heapGraph);
  