anchor.diff:      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.
  
positive.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
  
negative.code:     if (!_hasTimeline) return;
      // TODO: When NNBD is complete, delete the following line.
      ArgumentError.checkNotNull(name, 'name');
      if (!_isDartStreamEnabled()) {
        // Stream is disabled.
        return;
      }
      Map? instantArguments;
      if (arguments != null) {
        instantArguments = new Map.from(arguments);
  