DartDart_FileReadCallbackFunction typedef
        DartDart_FileReadCallbackFunction =
     void Function(Pointer<Pointer<Uint8> >  data, Pointer<IntPtr>  file_length, Pointer<Void>  stream)
    
    
    
Implementation
typedef DartDart_FileReadCallbackFunction = void Function(
    ffi.Pointer<ffi.Pointer<ffi.Uint8>> data,
    ffi.Pointer<ffi.IntPtr> file_length,
    ffi.Pointer<ffi.Void> stream);