【Flutter】Undefined name 'rootBundle'. Try correcting the name to one that is defined

final html = await rootBundle.loadString('assets/index.html');
final uri = Uri.dataFromString(
  html,
  mimeType: 'text/html',
  encoding: Encoding.getByName('utf-8'),
);

import 'package:flutter/services.dart' show rootBundle;
    
// 省略
final html = await rootBundle.loadString('assets/index.html');
final uri = Uri.dataFromString(
  html,
  mimeType: 'text/html',
  encoding: Encoding.getByName('utf-8'),
);