Allow translated messages that have no matching original message

pkg/intl/test/message_extraction/extract_to_json.dart

 #!/usr/bin/env dart
 // Copyright (c) 2013, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 /**
  * This script uses the extract_messages.dart library to find the Intl.message
  * calls in the target dart files and produces intl_messages.json containing the
  * information on those messages. It uses the analyzer-experimental parser
  * to find the information.
@@ skipping 16 matching lines @@
 import 'package:intl/src/intl_message.dart';
 import 'package:args/args.dart';
 
 main() {
   var args = new Options().arguments;
   var targetDir;
   var parser = new ArgParser();
   parser.addFlag("suppress-warnings", defaultsTo: false,
       callback: (x) => suppressWarnings = x);
 
-  void setTargetDir(value) => targetDir = value;
-
-  parser.addOption("output-dir", defaultsTo: '.', callback: setTargetDir);
+  parser.addOption("output-dir", defaultsTo: '.',
+      callback: (value) => targetDir = value);
   parser.parse(args);
   if (args.length == 0) {
     print('Usage: extract_to_json [--output-dir=<dir>] [files.dart]');
     print('Accepts Dart files and produces intl_messages.json');
     exit(0);
   }
   var allMessages = [];
   for (var arg in args.where((x) => x.contains(".dart"))) {
     var messages = parseFile(new File(arg));
     messages.forEach((k, v) => allMessages.add(toJson(v)));
@@ skipping 16 matching lines @@
  */
 Map toJson(IntlMessage message) {
   return {
     "name" : message.name,
     "description" : message.description,
     "message" : message.fullMessage(leaveTheInterpolationsInDartForm),
     "examples" : message.examples,
     "arguments" : message.arguments
   };
 }