How to build flutter web app in debug mode?

Try:

flutter build web --profile --dart-define=Dart2jsOptimization=O0

See:

https://github.com/flutter/flutter/blob/720dff6a94bd054e82ec4bf84b5cb802bbc52ddd/packages/flutter_tools/lib/src/build_system/targets/web.dart#L238-L239