Expose Service Describer in Dart mojo package

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

Index: mojo/dart/packages/mojo_services/lib/mojo/media/audio_server.mojom.dart
diff --git a/mojo/dart/packages/mojo_services/lib/mojo/media/audio_server.mojom.dart b/mojo/dart/packages/mojo_services/lib/mojo/media/audio_server.mojom.dart
index f9837270340170dd13d1e873a25a169b9e315940..2bc18a2ccfce6b4e4a1dd6967093353872045e88 100644
--- a/mojo/dart/packages/mojo_services/lib/mojo/media/audio_server.mojom.dart
+++ b/mojo/dart/packages/mojo_services/lib/mojo/media/audio_server.mojom.dart
@@ -92,11 +92,14 @@ const int _AudioServer_createTrackName = 0;
 
 
 class _AudioServerServiceDescription 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 AudioServer {
@@ -267,8 +270,13 @@ class AudioServerStub extends bindings.Stub {
 
   int get version => 0;
 
-  service_describer.ServiceDescription get serviceDescription =>
-    new _AudioServerServiceDescription();
+  static service_describer.ServiceDescription _cachedServiceDescription;
+  static service_describer.ServiceDescription get serviceDescription {
+    if (_cachedServiceDescription == null) {
+      _cachedServiceDescription = new _AudioServerServiceDescription();
+    }
+    return _cachedServiceDescription;
+  }
 }