anchor.diff:      }
       // Pop top item off of stack.
       var block = _stack.removeLast();
  +    if (block == null) {
  +      // Dart stream was disabled when start was called.
  +      return;
  +    }
       block._finish(arguments);
     }
   
  
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:   final TimelineTask? _parent;
    final String? _filterKey;
    final int _taskId;
    final List<_AsyncBlock?> _stack = [];
  }
  
  /// An asynchronous block of time on the timeline. This block can be kept
  