The issue is breaking every user that is using localizations and a custom FLUTTER_STORAGE_BASE_URL.

This also happens with flutter pub get if the l10n generation is synthetic.

The tool will report Found syntax errors but not any actual syntax errors.

Related issues:

• When run flutter gallery loaclly, faced with exception with Syntax Error when try to pub get. #117610
• flutter gen-i10n throws Found syntax errors. after upgrading to 3.7.0 (mostly affecting china mirror) #119435
• Flutter 3.7.0 Cause Localization Bug  #119585

Root cause

When FLUTTER_STORAGE_BASE_URL is being overridden, the cache getter will try to raise an error to STDERR that the user is using a custom storage base.

But using an error print will cause the generate tool recognized as an exception.

And when it's using synthetic package, flutter pub get will also raise exceptions.

Steps to Reproduce

1. Run export FLUTTER_STORAGE_BASE_URL=https://storage.flutter-io.cn in the terminal.
2. flutter create test_l10n
3. Setup internationalization for the project according to the doc.
4. flutter pub get or flutter gen-l10n

flutter doctor -v

Flutter assets will be downloaded from https://storage.flutter-io.cn. Make sure you trust this source!

[!] 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