如何给已有项目增加更多平台的支持呢？

flutter create --platforms=windows,macos,linux .

可以参考官方文档：

https://docs.flutter.dev/desktop#add-desktop-support-to-an-existing-flutter-app

如果你用的版本是比较低的，你最好开启dev channel

flutter channel dev
flutter upgrade
flutter config --enable-linux-desktop
flutter config --enable-macos-desktop
flutter config --enable-windows-desktop