anchor.diff:        VariableModel<Variable, Type> variableModel) {
       variableInfo[variable] = variableModel;
     }
  +
  +  @override
  +  String toString() => 'VariableReference($variable)';
   
     @override
     VariableModel<Variable, Type>? _getInfo(
  
positive.code:       _wrapped._dumpState();
      }
      if (isQuery) {
        print('  => $result');
      }
      return result;
    }
  }
  
  
negative.code: class FlowAnalysisDebug<Node extends Object, Statement extends Node,
          Expression extends Object, Variable extends Object, Type extends Object>
      implements FlowAnalysis<Node, Statement, Expression, Variable, Type> {
    static int _nextCallbackId = 0;
  
    static Expando<String> _description = new Expando<String>();
  
    FlowAnalysis<Node, Statement, Expression, Variable, Type> _wrapped;
  
    bool _exceptionOccurred = false;
  