Description

Today, the flutter build mode is driven by the $CONFIGURATION from Xcode. This is easy to mess up if you're using a local engine, and the error message in such a case isn't helpful.

This change writes the FLUTTER_BUILD_MODE flag for the engine you're using. We can't use the ambient build mode because it doesn't necessarily correspond to the engine build type.

Related Issues

Fixes #51673

Tests

I added the following tests:

Test that we write the right flags into Generated.xcconfig and the other new file.

Checklist

Before you create this PR confirm that it meets all requirements listed below by checking the relevant checkboxes ([x]). This will ensure a smooth and quick review process.

• I read the Contributor Guide and followed the process outlined there for submitting PRs.
• I signed the CLA.
• I read and followed the Flutter Style Guide, including Features we expect every widget to implement.
• I read the Tree Hygiene wiki page, which explains my responsibilities.
• I updated/added relevant documentation (doc comments with ///).
• All existing and new tests are passing.
• The analyzer (flutter analyze --flutter-repo) does not report any problems on my PR.
• I am willing to follow-up on review comments in a timely manner.

Breaking Change

Did any tests fail when you ran them? Please read Handling breaking changes.

• No, no existing tests failed, so this is not a breaking change.