The FLUTTER_* variables defined in Generated.xcconfig are critical to successfully building for iOS. These are used in at least two key places:

1. To locate the the xcode_backend.sh file in the build run-script phase.
2. Within of xcode_backend.sh, for the various Flutter build settings.

We should detect that this file is (transitively) included from the build configuration's .xcconfig. One way to do this might just be to use xcodebuild -showBuildSettings and verify that all required variables are set. That avoids parsing Xcode project files and tying ourselves to a proprietary file format that could change.