Flutter widget test error: OS Error: Read-only file system, errno = 30

when I use “flutter run xxx_test.dart”, I got an error:

I/flutter (15574): The following FileSystemException was thrown running a test:

I/flutter (15574): Creation failed, path = '//.test' (OS Error: Read-only file system, errno = 30)

since I call function:

dir.createSync(recursive: true);

how to resolve it? thx