Implement missing interface member in mock ModelDB

This fixes an analyzer warning

BUG=
R=kustermann@google.com

Review URL: https://codereview.chromium.org//2261453002 .
diff --git a/CHANGELOG.md b/CHANGELOG.md
index eb561f1..70e5767 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -1,3 +1,7 @@
+## 0.2.0+14
+
+* Fix analyzer warning.
+
 ## 0.2.0+13
 
 * Remove crypto dependency and upgrade dart dependency to >=1.13 since
diff --git a/pubspec.yaml b/pubspec.yaml
index e5a7907..29df7fb 100644
--- a/pubspec.yaml
+++ b/pubspec.yaml
@@ -1,5 +1,5 @@
 name: gcloud
-version: 0.2.0+13
+version: 0.2.0+14
 author: Dart Team <misc@dartlang.org>
 description: Dart gcloud APIs
 homepage: https://github.com/dart-lang/gcloud
diff --git a/test/db/properties_test.dart b/test/db/properties_test.dart
index d0ae074..eed59e9 100644
--- a/test/db/properties_test.dart
+++ b/test/db/properties_test.dart
@@ -238,4 +238,5 @@
   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;
 }