anchor.diff:    final TimelineTask? _parent;
     final String? _filterKey;
     final int _taskId;
  -  final List<_AsyncBlock> _stack = [];
  +  final List<_AsyncBlock?> _stack = [];
   }
   
   /// An asynchronous block of time on the timeline. This block can be kept
  
positive.code:     if (!_hasTimeline) return;
      // TODO: When NNBD is complete, delete the following line.
      ArgumentError.checkNotNull(name, 'name');
      Map? instantArguments;
      if (arguments != null) {
        instantArguments = new Map.from(arguments);
  
negative.code:     if (!_hasTimeline) return;
      // TODO: When NNBD is complete, delete the following line.
      ArgumentError.checkNotNull(name, 'name');
      if (!_isDartStreamEnabled()) {
        // Push a null onto the stack and return.
        _stack.add(null);
        return;
      }
      var block = new _AsyncBlock._(name, _taskId);
      _stack.add(block);
      // TODO(39115): Spurious error about collection literal ambiguity.
  