Fix typo in id-generator.

pkg/dart_messages/bin/message_id.dart

 // Copyright (c) 2015, 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.
 
 import 'dart:math' as math;
 
 import '../lib/shared_messages.dart' as shared_messages;
 
 math.Random random = new math.Random();
 
@@ skipping 12 matching lines @@
 /// Computes a random message ID that hasn't been used before.
 void main() {
   var usedIds =
       shared_messages.MESSAGES.values.map((entry) => entry.id).toSet();
 
   print("${usedIds.length} existing ids");
 
   var newId;
   do {
     newId = computeId();
-  } while (!usedIds.contains(newId));
+  } while (usedIds.contains(newId));
   print("Available id: $newId");
 }