successors property Null safety

Implementation

Iterable<HeapSnapshotObject> get successors sync* {
  final startSuccessorIndex = _graph._firstSuccessors[_oid];
  final limitSuccessorIndex = _graph._firstSuccessors[_oid + 1];

  for (int nextSuccessorIndex = startSuccessorIndex;
      nextSuccessorIndex < limitSuccessorIndex;
      ++nextSuccessorIndex) {
    final successorId = _graph._successors[nextSuccessorIndex];
    yield _graph.objects[successorId];
  }
}