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:   final ErrorReporter _errorReporter;
  
    /// The object used to track the usage of labels within a given label scope.
    _LabelTracker? _labelTracker;
  
    DeadCodeVerifier(this._errorReporter);
  
    @override
    void visitBreakStatement(BreakStatement node) {
      _labelTracker?.recordUsage(node.label?.name);
    }
  
    @override
    void visitContinueStatement(ContinueStatement node) {
      _labelTracker?.recordUsage(node.label?.name);
    }
  
    @override
  