
This PR adds a new flag `default-flavor` in the `flutter` section of `pubspec.yaml`. It allows developers of multi-flavor android apps to specify a default flavor to be used for `flutter run`, `flutter build` etc. Using `flutter run` on flavored apps already works without specifying `--flavor` already works on iOS (it defaults to the `runner` schema), so I (and others in #22856) figured this would be nice to have. fixes #22856