Allow translated messages that have no matching original message

pkg/intl/test/message_extraction/generate_from_json.dart

Index: pkg/intl/test/message_extraction/generate_from_json.dart
diff --git a/pkg/intl/test/message_extraction/generate_from_json.dart b/pkg/intl/test/message_extraction/generate_from_json.dart
index 0759507878a8f68d4b60ae75b8e2b6b69caecdaa..c991a44421e3133ea6948a60e8d8bcd9ca3f1b13 100644
--- a/pkg/intl/test/message_extraction/generate_from_json.dart
+++ b/pkg/intl/test/message_extraction/generate_from_json.dart
@@ -43,8 +43,8 @@ main() {
   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);
   }