[gen-l10n] Should not use dart format for the untranslated message file #119594
Description
...because it cannot be formatted by dart format, which will cause the generation failed.
This also blocks users that used to have untranslated fields after they upgraded to Flutter 3.7.
[!] Flutter (Channel stable, 3.7.0, on Microsoft Windows [版本 10.0.22000.1455], locale zh-CN)
• Flutter version 3.7.0 on channel stable at X:\SDK\flutter
• Upstream repository https://github.com/flutter/flutter.git
• Framework revision b06b8b2710 (7 days ago), 2023-01-23 16:55:55 -0800
• Engine revision b24591ed32
• Dart version 2.19.0
• DevTools version 2.20.1
• Pub download mirror https://pub.flutter-io.cn
• Flutter download mirror https://storage.flutter-io.cn