anchor.diff:        var index = labelMap[labelName];
         if (index != null) {
           used[index] = true;
  -      } else if (outerTracker != null) {
  -        outerTracker!.recordUsage(labelName);
  +      } else {
  +        outerTracker?.recordUsage(labelName);
         }
       }
     }
  
positive.code: 
    @override
    void visitLabeledStatement(LabeledStatement node) {
      _pushLabels(node.labels);
      try {
        super.visitLabeledStatement(node);
      } finally {
        _popLabels();
      }
    }
  
    @override
  
negative.code:     // So, they look unreachable, but this does not make sense.
      if (node is Comment) return;
  
      var flowAnalysis = _flowAnalysis;
      if (flowAnalysis == null) return;
      flowAnalysis.checkUnreachableNode(node);
  
      var flow = flowAnalysis.flow;
      if (flow == null) return;
  
      if (flow.isReachable) return;
  