UnixFileDispatcherImpl contains close0(), dup0(), and preClose0() native methods which are commented as:

// Shared with SocketDispatcher and DatagramDispatcher but
// NOT used by FileDispatcherImpl

These should be moved to a place more appropriate for SocketDispatcher and DatagramDispatcher. UnixFileDispatcherImpl also contains preClose() and dup() methods which are not called anywhere hence could be removed.