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..ffaec050a3b940c458ddb6b513770c8e67400d06 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 {
@@ -1110,8 +1113,13 @@ class MediaSinkStub extends bindings.Stub {
 
   int get version => 0;
 
-  service_describer.ServiceDescription get serviceDescription =>
-    new _MediaSinkServiceDescription();
+  static service_describer.ServiceDescription _cachedServiceDescription;
+  static service_describer.ServiceDescription get serviceDescription {
+    if (_cachedServiceDescription == null) {
+      _cachedServiceDescription = new _MediaSinkServiceDescription();
+    }
+    return _cachedServiceDescription;
+  }
 }