Implement missing interface member in mock ModelDB

test/db/properties_test.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.properties_test;
 
 import 'dart:typed_data';
 
 import 'package:gcloud/db.dart';
 import 'package:gcloud/datastore.dart' as datastore;
@@ skipping 220 matching lines @@
      throw "Broken test";
     }
     return _datastoreKey;
   }
 
   Map<String, Property> propertiesForModel(modelDescription) => null;
   Model fromDatastoreEntity(datastore.Entity entity) => null;
   datastore.Entity toDatastoreEntity(Model model) => null;
   String fieldNameToPropertyName(String kind, String fieldName) => null;
   String kindName(Type type) => null;
+  Object toDatastoreValue(String kind, String fieldName, Object value) => null;
 }