main.dart

import 'package:flutter/material.dart';

void main() => runApp(const MyApp());

class MyApp extends StatelessWidget {
  const MyApp({Key? key}) : super(key: key);

  static const String _title = 'AppBarのtitleを改行する';

  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      debugShowCheckedModeBanner: false,
      title: _title,
      home: Scaffold(
        appBar: AppBar(
          title: Column(
            children: const [
              Text(
                '1行目のタイトル',
                style: TextStyle(
                  color: Colors.red,
                  fontSize: 14.0,
                ),
              ),
              Text(
                '2行目のタイトル',
                style: TextStyle(
                  color: Colors.white,
                ),
              ),
            ],
          ),
        ),
        body: const Center(
          child: Text('demo'),
        ),
      ),
    );
  }
}