Expose Service Describer in Dart mojo package

mojo/dart/packages/mojo_services/lib/mojo/media/media_factory.mojom.dart

Index: mojo/dart/packages/mojo_services/lib/mojo/media/media_factory.mojom.dart
diff --git a/mojo/dart/packages/mojo_services/lib/mojo/media/media_factory.mojom.dart b/mojo/dart/packages/mojo_services/lib/mojo/media/media_factory.mojom.dart
index ea2e76225a5a34a60eb18638273b058f28634170..7bbcd5a45aab8bfcdc15e25db67c9b2a93326457 100644
--- a/mojo/dart/packages/mojo_services/lib/mojo/media/media_factory.mojom.dart
+++ b/mojo/dart/packages/mojo_services/lib/mojo/media/media_factory.mojom.dart
@@ -327,11 +327,14 @@ const int _MediaFactory_createSinkName = 2;
 
 
 class _MediaFactoryServiceDescription 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 MediaFactory {
@@ -537,8 +540,13 @@ class MediaFactoryStub extends bindings.Stub {
 
   int get version => 0;
 
-  service_describer.ServiceDescription get serviceDescription =>
-    new _MediaFactoryServiceDescription();
+  static service_describer.ServiceDescription _cachedServiceDescription;
+  static service_describer.ServiceDescription get serviceDescription {
+    if (_cachedServiceDescription == null) {
+      _cachedServiceDescription = new _MediaFactoryServiceDescription();
+    }
+    return _cachedServiceDescription;
+  }
 }