wakelockPlatformInstance top-level property Null safety
This needs to be exposed for testing as unit tests might run on macOS.
In that case, the "hacky" instance override that we use here would be
triggered for the unit tests, even though the unit tests should actually
pigeon method channel implementation. Therefore, we want to
override this in tests that run on macOS (where there is no actual device).
@visibleForTesting var wakelockPlatformInstance = _defaultPlatformInstance;