Fix metamodel.dart: Use model's static member

lib/db/metamodel.dart

Index: lib/db/metamodel.dart
diff --git a/lib/db/metamodel.dart b/lib/db/metamodel.dart
index 9571066c6cd32e3d0ebcda04da3c071aaa5bfd55..81ff5b766a411e2111ed376990af5b9ad6a975b5 100644
--- a/lib/db/metamodel.dart
+++ b/lib/db/metamodel.dart
@@ -12,7 +12,7 @@ class Namespace extends db.ExpandoModel {
 
   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;
   }
 }