Expose Service Describer in Dart mojo package

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

Index: mojo/dart/packages/mojo_services/lib/mojo/media/media_pipe.mojom.dart
diff --git a/mojo/dart/packages/mojo_services/lib/mojo/media/media_pipe.mojom.dart b/mojo/dart/packages/mojo_services/lib/mojo/media/media_pipe.mojom.dart
index 581eb0e5ab38236bc86f92bd4493457c8fffbb1b..a367aa6e996d5edbf33304198e20bcee0709a1c4 100644
--- a/mojo/dart/packages/mojo_services/lib/mojo/media/media_pipe.mojom.dart
+++ b/mojo/dart/packages/mojo_services/lib/mojo/media/media_pipe.mojom.dart
@@ -807,11 +807,14 @@ class MediaPipeSendResult extends bindings.MojoEnum {
 
 
 class _MediaPipeServiceDescription 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 MediaPipe {
@@ -1135,8 +1138,13 @@ class MediaPipeStub extends bindings.Stub {
 
   int get version => 0;
 
-  service_describer.ServiceDescription get serviceDescription =>
-    new _MediaPipeServiceDescription();
+  static service_describer.ServiceDescription _cachedServiceDescription;
+  static service_describer.ServiceDescription get serviceDescription {
+    if (_cachedServiceDescription == null) {
+      _cachedServiceDescription = new _MediaPipeServiceDescription();
+    }
+    return _cachedServiceDescription;
+  }
 }