Allow translated messages that have no matching original message

pkg/intl/test/message_extraction/generate_from_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.
 
 /**
  * A main program that takes as input a source Dart file and a number
  * of JSON files representing translations of messages from the corresponding
  * Dart file. See extract_to_json.dart and make_hardcoded_translation.dart.
  *
@@ skipping 25 matching lines @@
   var parser = new ArgParser();
   parser.addFlag("suppress-warnings", defaultsTo: false,
       callback: (x) => suppressWarnings = x);
   parser.addOption("output-dir", defaultsTo: '.',
       callback: (x) => targetDir = x);
   parser.addOption("generated-file-prefix", defaultsTo: '',
       callback: (x) => generatedFilePrefix = x);
   parser.parse(args);
   if (args.length == 0) {
     print('Usage: generate_from_json [--output-dir=<dir>]'
-        ' [generated-file-prefix=<prefix>] file1.dart file2.dart ...'
-        ' outputFile.json');
+        ' [--generated-file-prefix=<prefix>] file1.dart file2.dart ...'
+        ' translation1.json translation2.json ...');
     exit(0);
   }
 
   var dartFiles = args.where((x) => x.endsWith("dart")).toList();
   var jsonFiles = args.where((x) => x.endsWith(".json")).toList();
 
   // We're re-parsing the original files to find the corresponding messages,
   // so if there are warnings extracting the messages, suppress them.
   suppressWarnings = true;
   var allMessages = dartFiles.map((each) => parseFile(new File(each)));
@@ skipping 41 matching lines @@
   BasicTranslatedMessage(String name, String translated) :
       super(name, translated);
 
   IntlMessage get originalMessage =>
       (super.originalMessage == null) ? _findOriginal() : super.originalMessage;
 
   // We know that our [id] is the name of the message, which is used as the
   //key in [messages].
   IntlMessage _findOriginal() => originalMessage = messages[id];
 }