Expose Service Describer in Dart mojo package

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

Index: mojo/dart/packages/mojo_services/lib/mojo/media/media_sink.mojom.dart
diff --git a/mojo/dart/packages/mojo_services/lib/mojo/media/media_sink.mojom.dart b/mojo/dart/packages/mojo_services/lib/mojo/media/media_sink.mojom.dart
index 5f015879cdcedf2c69dd0196d7ce8bc9c0c3c19f..a25f90bd4e679cc4c57c40bd8a33b94852dff464 100644
--- a/mojo/dart/packages/mojo_services/lib/mojo/media/media_sink.mojom.dart
+++ b/mojo/dart/packages/mojo_services/lib/mojo/media/media_sink.mojom.dart
@@ -763,11 +763,14 @@ const int _MediaSink_pauseName = 6;
 
 
 class _MediaSinkServiceDescription 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 MediaSink {
@@ -1110,7 +1113,7 @@ class MediaSinkStub extends bindings.Stub {
 
   int get version => 0;
 
-  service_describer.ServiceDescription get serviceDescription =>
+  static service_describer.ServiceDescription get serviceDescription =>
     new _MediaSinkServiceDescription();
 }