Fix metamodel.dart: Use model's static member

R=sgjesse@google.com

Review URL: https://codereview.chromium.org//696263002
diff --git a/pkgs/gcloud/lib/db/metamodel.dart b/pkgs/gcloud/lib/db/metamodel.dart
index 9571066..81ff5b7 100644
--- a/pkgs/gcloud/lib/db/metamodel.dart
+++ b/pkgs/gcloud/lib/db/metamodel.dart
@@ -12,7 +12,7 @@
 
   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;
   }
 }