Update the protocol buffer compiler plugin to use Int64 from the fixnum package

prepend.dart

 #!/usr/bin/env dart
 import 'dart:io';
 
 main() {
   final fileName = new Options().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');
 }