class FileIOException implements Exception {
 const FileIOException([String this.message = "",
                        OSError this.osError = null]);
 String toString() {
   StringBuffer sb = new StringBuffer();
   sb.write("FileIOException");
   if (!message.isEmpty) {
     sb.write(": $message");
     if (osError != null) {
       sb.write(" ($osError)");
     }
   } else if (osError != null) {
     sb.write(": osError");
   }
   return sb.toString();
 }
 final String message;
 final OSError osError;
}