Throw a StateError in case no model was registered for a given kind

lib/src/db/model_db_impl.dart

Index: lib/src/db/model_db_impl.dart
diff --git a/lib/src/db/model_db_impl.dart b/lib/src/db/model_db_impl.dart
index 934242279ad5339ef500977bf2c4e4bbf791ef38..dc26f134eeb2601e6f39ccdb8340fc3d490079ca 100644
--- a/lib/src/db/model_db_impl.dart
+++ b/lib/src/db/model_db_impl.dart
@@ -61,7 +61,12 @@ class ModelDBImpl implements ModelDB {
     Key key = namespace.emptyKey;
     for (var element in datastoreKey.elements) {
       var type = _type2ModelDesc[_kind2ModelDesc[element.kind]];
-      assert (type != null);
+      if (type == null) {
+        throw new StateError(
+            'Could not find a model associated with kind "${element.kind}". '
+            'Please ensure a model class was annotated with '
+            '`@Kind(name: "${element.kind}")`.');
+      }
       key = key.append(type, id: element.id);
     }
     return key;