thanks for your article and pointing to the article by verygood.ventures.
I will give the robot pattern a try.
I also tried out your code in integration_test.dart but only get the content 'null' in integration_response_data.json.
The same is described in the ticket https://github.com/flutter/flutter/issues/74391 .
Did you ever get a useful result in this file?
And if yes, what is more to do than using the code in integration_test.dart?
BTW: in your github repo this file contains also only the string 'null' :-)