The strict mode policy ensures that the engine doesn't read or write to disk in a typical Flutter app. flutter/engine@dc93db5#diff-6c316cfa52e434f5c02f5921476fd3f8 added this policy.
However, this policy affects all tests including those that take screenshot tests. As a result, screenshot test cannot save the screenshot to disk.

A workaround could be to launch a separate activity that sets the policy, and then the test verifies that the app doesn't crash.