On Android, when there are multiple flavors defined, and the user runs the app but does not specify a value for the --flavor flag then the following error is shown:

You must specify a --flavor option to select one of them.

This is implemented like that here: https://github.com/flutter/flutter/blob/master/packages/flutter_tools/lib/src/android/gradle.dart#L342

This makes it so that everytime we have to specifiy a value for --flavor.
I would propose when the flag is not specified to just default to the first one.
This will make it faster to start your Flutter app, not having to write --flavor _flavor_ everytime.