Update Dart protoc plugin to latest SDK

prepend.dart

 #!/usr/bin/env dart
 import 'dart:io';
 
-main() {
-  final fileName = new Options().arguments.first;
+main(List<String> arguments) {
+  final fileName = arguments.first;
   final file = new File(fileName);
   final content = file.readAsStringSync();
   file.writeAsStringSync('#!/usr/bin/env dart\n$content');
   // For development use the line below instead of the one above to
   // avoid using the dart2dart code.
   // file.writeAsStringSync('#!/bin/bash\ndart bin/protoc_plugin.dart');
 }