anchor.diff:        vmService.HeapSnapshotObject o,
         vmService.HeapSnapshotGraph graph,
         Map<Uri, Map<String, List<String>>> prettyPrints) {
  -    if (o.classId == 0) {
  +    if (o.classId <= 0) {
         return "Class sentinel";
       }
  -    vmService.HeapSnapshotClass class_ = graph.classes[o.classId - 1];
  +    vmService.HeapSnapshotClass class_ = o.klass;
   
       if (class_.name == "_OneByteString") {
         return '"${o.data}"';
  
positive.code:       vmService.HeapSnapshotGraph graph) {
      for (vmService.HeapSnapshotField field in class_.fields) {
        if (field.name == name) {
          int index = o.references[field.index] - 1;
          if (index < 0) {
            // Sentinel object.
            return null;
  
negative.code:         new List<bool>.filled(graph.classes.length, false);
      for (int i = 0; i < graph.objects.length; i++) {
        vmService.HeapSnapshotObject o = graph.objects[i];
        if (o.classId <= 0) {
          // Sentinel.
          continue;
        }
  