Update Dart protoc plugin to latest SDK

prepend.dart

Index: prepend.dart
diff --git a/prepend.dart b/prepend.dart
index 397cfc56aab4ebabaeed2a9c5c2d141cb67a28f6..5ca276f3a5d32f18bc20c2d88cf5568791a898d4 100755
--- a/prepend.dart
+++ b/prepend.dart
@@ -1,8 +1,8 @@
 #!/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');