Expose Service Describer in Dart mojo package

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

Index: mojo/dart/packages/mojo_services/lib/mojo/media/audio_track.mojom.dart
diff --git a/mojo/dart/packages/mojo_services/lib/mojo/media/audio_track.mojom.dart b/mojo/dart/packages/mojo_services/lib/mojo/media/audio_track.mojom.dart
index 4e95a3fc5556967675929517e154d8ee019e0375..127421dfe9d96ee3d275f01833d9bfed35341382 100644
--- a/mojo/dart/packages/mojo_services/lib/mojo/media/audio_track.mojom.dart
+++ b/mojo/dart/packages/mojo_services/lib/mojo/media/audio_track.mojom.dart
@@ -601,11 +601,14 @@ const int _AudioTrack_setGainName = 3;
 
 
 class _AudioTrackServiceDescription 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 AudioTrack {
@@ -865,8 +868,13 @@ class AudioTrackStub extends bindings.Stub {
 
   int get version => 0;
 
-  service_describer.ServiceDescription get serviceDescription =>
-    new _AudioTrackServiceDescription();
+  static service_describer.ServiceDescription _cachedServiceDescription;
+  static service_describer.ServiceDescription get serviceDescription {
+    if (_cachedServiceDescription == null) {
+      _cachedServiceDescription = new _AudioTrackServiceDescription();
+    }
+    return _cachedServiceDescription;
+  }
 }