Text(
    'hello',
    textDirection: TextDirection.ltr,
    style: TextStyle(
        fontSize: 40.0,
        fontWeight: FontWeight.bold,
        color: Colors.yellow,
    ),
)

完整代码：

import 'package:flutter/material.dart';

void main() => runApp(App());

class App extends StatelessWidget {
    @override
    Widget build(BuildContext context) {
        return Center(
            child: Text(
                'hello',
                textDirection: TextDirection.ltr,
                style: TextStyle(
                    fontSize: 40.0,
                    fontWeight: FontWeight.bold,
                    color: Colors.yellow,
                ),
            ),
        );
    }
}