Expose Service Describer in Dart mojo package

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

Index: mojo/dart/packages/mojo_services/lib/mojo/media/media_source.mojom.dart
diff --git a/mojo/dart/packages/mojo_services/lib/mojo/media/media_source.mojom.dart b/mojo/dart/packages/mojo_services/lib/mojo/media/media_source.mojom.dart
index 33602c38fee82fbc79dccc1e1ee3689784e46a32..908351129a10045dc124e3515f49a7f4546adafa 100644
--- a/mojo/dart/packages/mojo_services/lib/mojo/media/media_source.mojom.dart
+++ b/mojo/dart/packages/mojo_services/lib/mojo/media/media_source.mojom.dart
@@ -1117,11 +1117,14 @@ const int _MediaSource_prepareName = 7;
 
 
 class _MediaSourceServiceDescription 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 MediaSource {
@@ -1564,8 +1567,13 @@ class MediaSourceStub extends bindings.Stub {
 
   int get version => 0;
 
-  service_describer.ServiceDescription get serviceDescription =>
-    new _MediaSourceServiceDescription();
+  static service_describer.ServiceDescription _cachedServiceDescription;
+  static service_describer.ServiceDescription get serviceDescription {
+    if (_cachedServiceDescription == null) {
+      _cachedServiceDescription = new _MediaSourceServiceDescription();
+    }
+    return _cachedServiceDescription;
+  }
 }