This is an abbreviated project to demonstrate a bug in AGP 3.5.0's Lint functionality.
The core of the issue is that using @VisibleForTesting(otherwise = VisibleForTesting.PACKAGE_PRIVATE) in a Kotlin file is not applied correctly. All package-private attempts to access those classes fail lint.
To see the error, use ./gradlew :engine:lintDebug
Note: The same VisibleForTesting declaration works if it is declared in Java. See the difference between the WorkingEngine* and NotWorkingEngine* classes.
Note2: Android Studio does not display the warning, leading me to think that this applies to the non-IDE invocation of Lint only.