Expose Service Describer in Dart mojo package

mojo/dart/packages/mojo_services/lib/mojo/service_registry.mojom.dart

Index: mojo/dart/packages/mojo_services/lib/mojo/service_registry.mojom.dart
diff --git a/mojo/dart/packages/mojo_services/lib/mojo/service_registry.mojom.dart b/mojo/dart/packages/mojo_services/lib/mojo/service_registry.mojom.dart
index 542703a7d80816e2d9b02c3372412673ca60345e..c555ee614d8ca17d8d718476f2262e777ab770df 100644
--- a/mojo/dart/packages/mojo_services/lib/mojo/service_registry.mojom.dart
+++ b/mojo/dart/packages/mojo_services/lib/mojo/service_registry.mojom.dart
@@ -120,11 +120,14 @@ const int _ServiceRegistry_addServicesName = 0;
 
 
 class _ServiceRegistryServiceDescription implements service_describer.ServiceDescription {
-  dynamic getTopLevelInterface([Function responseFactory]) => null;
+  dynamic getTopLevelInterface([Function responseFactory]) =>
+      responseFactory(null);
 
-  dynamic getTypeDefinition(String typeKey, [Function responseFactory]) => null;
+  dynamic getTypeDefinition(String typeKey, [Function responseFactory]) =>
+      responseFactory(null);
 
-  dynamic getAllTypeDefinitions([Function responseFactory]) => null;
+  dynamic getAllTypeDefinitions([Function responseFactory]) =>
+      responseFactory(null);
 }
 
 abstract class ServiceRegistry {
@@ -296,8 +299,13 @@ class ServiceRegistryStub extends bindings.Stub {
 
   int get version => 0;
 
-  service_describer.ServiceDescription get serviceDescription =>
-    new _ServiceRegistryServiceDescription();
+  static service_describer.ServiceDescription _cachedServiceDescription;
+  static service_describer.ServiceDescription get serviceDescription {
+    if (_cachedServiceDescription == null) {
+      _cachedServiceDescription = new _ServiceRegistryServiceDescription();
+    }
+    return _cachedServiceDescription;
+  }
 }