Pourquoi je ne peux pas faire une chaîne de classe comme domaine de domaine pour la carte de tarte dans graphs_flutter

    import 'package:charts_flutter/flutter.dart' as charts;
import 'package:flutter/material.dart';

class PieOutsideLabelChart extends StatelessWidget {
  final List<charts.Series> seriesList;
  final bool animate;

  PieOutsideLabelChart(this.seriesList, {this.animate});

  factory PieOutsideLabelChart.withSampleData() {
    return new PieOutsideLabelChart(
      _createSampleData(),
      animate: false,
    );
  }
  @override
  Widget build(BuildContext context) {
    return new charts.PieChart(seriesList,
        animate: animate,
        defaultRenderer: new charts.ArcRendererConfig(arcRendererDecorators: [
          new charts.ArcLabelDecorator(
              labelPosition: charts.ArcLabelPosition.outside)
        ]));
  }

  static List<charts.Series<LinearSales, int>> _createSampleData() {
    final data = [
      new LinearSales('a long text', 100),
      new LinearSales('even longer text', 75),
      new LinearSales('i am long text', 25),
      new LinearSales('the longest text ever', 5),
    ];
    return [
      new charts.Series<LinearSales, int>(
        id: 'Sales',
        domainFn: (LinearSales sales, _) => sales.year,
        measureFn: (LinearSales sales, _) => sales.sales,
        data: data,
        labelAccessorFn: (LinearSales row, _) => '${row.year}: ${row.sales}',
      )
    ];
  }
}
class LinearSales {
  final String year;
  final int sales;

  LinearSales(this.year, this.sales);
}
main(List<String> args) {
  runApp(MaterialApp(
    title: 'charts',
    home: new Scaffold(
      appBar: new AppBar(title: new Text("نمودار فلان"),),
      body:new ListView(children: <Widget>[
          new Card
          (child: PieOutsideLabelChart(PieOutsideLabelChart._createSampleData()),)
        //   ]
        //  ),
      ]
    )
  )
   )
    );
}