Fix metamodel.dart: Use model's static member

lib/db/metamodel.dart

 // Copyright (c) 2014, 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.
 
 library gcloud.db.meta_model;
 
 import '../db.dart' as db;
 
 @db.Kind(name: '__namespace__')
 class Namespace extends db.ExpandoModel {
   static const int EmptyNamespaceId = 1;
 
   String get name {
     // The default namespace will be reported with id 1.
-    if (id == NamespaceDescription.EmptyNamespaceId) return null;
+    if (id == Namespace.EmptyNamespaceId) return null;
     return id;
   }
 }
 
 @db.Kind(name: '__kind__')
 class Kind extends db.Model {
   String get name => id;
 }