anchor.diff:  
     @override
     void visitChildren(AstVisitor visitor) {
  -    _safelyVisitChild(_variableList, visitor);
  +    _variableList?.accept(visitor);
     }
   }
   
  
positive.code:   @override
    void visitChildren(AstVisitor visitor) {
      labels.accept(visitor);
      _safelyVisitChild(_expression, visitor);
      statements.accept(visitor);
    }
  }
  
negative.code: 
    @override
    void visitChildren(AstVisitor visitor) {
      _expression?.accept(visitor);
    }
  }
  
  