anchor.diff:    }
   
     @override
  -  void _visit(
  -      Harness h, FlowAnalysis<Node, Statement, Expression, Var, Type> flow) {
  -    flow.functionExpression_begin(this);
  -    body._visit(h, flow);
  -    flow.functionExpression_end();
  +  void _visit(Harness h) {
  +    h._flow.functionExpression_begin(this);
  +    body._visit(h);
  +    h._flow.functionExpression_end();
     }
   }
   
  
positive.code:   }
  
    @override
    Type _visit(
        Harness h, FlowAnalysis<Node, Statement, Expression, Var, Type> flow) {
      operand._visit(h, flow);
      flow.handleExit();
      return Type('Never');
    }
  }
  
negative.code:   void _preVisit(AssignedVariables<Node, Var> assignedVariables,
        {_LValueDisposition disposition});
  
    void _visitWrite(Harness h, Expression assignmentExpression, Type writtenType,
        Expression? rhs);
  }
  
  /// Representation of an expression or statement in the pseudo-Dart language
  