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