anchor.diff:    void _preVisit(AssignedVariables<Node, Var> assignedVariables) {}
   
     @override
  -  void _visit(
  -      Harness h, FlowAnalysis<Node, Statement, Expression, Var, Type> flow) {
  -    expect(flow.isReachable, expectedReachable);
  +  void _visit(Harness h) {
  +    expect(h._flow.isReachable, expectedReachable);
     }
   }
   
  
positive.code:   }
  
    @override
    Type _visit(
        Harness h, FlowAnalysis<Node, Statement, Expression, Var, Type> flow) {
      var lhsType = lhs._visit(h, flow);
      flow.ifNullExpression_rightBegin(lhs, lhsType);
      var rhsType = rhs._visit(h, flow);
      flow.ifNullExpression_end();
      return h._lub(h.promoteToNonNull(lhsType), rhsType);
    }
  }
  
negative.code:   void _preVisit(AssignedVariables<Node, Var> assignedVariables) {}
  
    @override
    void _visit(Harness h) {
      var promotedType = h._flow.promotedType(variable);
      expect(promotedType?.type, expectedTypeStr, reason: '$_creationTrace');
    }
  }
  