Expose Service Describer in Dart mojo package

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

Index: mojo/dart/packages/mojo_services/lib/mojo/media/media_player.mojom.dart
diff --git a/mojo/dart/packages/mojo_services/lib/mojo/media/media_player.mojom.dart b/mojo/dart/packages/mojo_services/lib/mojo/media/media_player.mojom.dart
index efc90ea97f9ea0fb34aadeb769a99a8d5dde8539..60e6774d04f69c30c27b5e5ca81fe88ca95dce3a 100644
--- a/mojo/dart/packages/mojo_services/lib/mojo/media/media_player.mojom.dart
+++ b/mojo/dart/packages/mojo_services/lib/mojo/media/media_player.mojom.dart
@@ -414,11 +414,14 @@ const int _MediaPlayer_getStatusName = 2;
 
 
 class _MediaPlayerServiceDescription 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 MediaPlayer {
@@ -660,8 +663,13 @@ class MediaPlayerStub extends bindings.Stub {
 
   int get version => 0;
 
-  service_describer.ServiceDescription get serviceDescription =>
-    new _MediaPlayerServiceDescription();
+  static service_describer.ServiceDescription _cachedServiceDescription;
+  static service_describer.ServiceDescription get serviceDescription {
+    if (_cachedServiceDescription == null) {
+      _cachedServiceDescription = new _MediaPlayerServiceDescription();
+    }
+    return _cachedServiceDescription;
+  }
 }