anchor.diff:      node.accept(_AssignedVariablesVisitor(assignedVariables));
   
       return FlowAnalysisHelper._(
  -        _NodeOperations(),
  +        const AnalyzerNodeOperations(),
           _TypeSystemTypeOperations(typeSystem),
           assignedVariables,
           retainDataForTesting ? FlowAnalysisResult() : null);
  
positive.code:   }
  }
  
  class _NodeOperations implements NodeOperations<Expression> {
    @override
    Expression unwrapParenthesized(Expression node) {
      return node.unParenthesized;
    }
  }
  
  class _TypeSystemTypeOperations
      implements TypeOperations<VariableElement, DartType> {
    final TypeSystem typeSystem;
  
negative.code:       flow = FlowAnalysis<Statement, Expression, VariableElement, DartType>(
          _nodeOperations,
          _typeOperations,
          AnalyzerFunctionBodyAccess(node),
        );
      }
  
  