Expose Service Describer in Dart mojo package

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

Index: mojo/dart/packages/mojo_services/lib/mojo/media/rate_control.mojom.dart
diff --git a/mojo/dart/packages/mojo_services/lib/mojo/media/rate_control.mojom.dart b/mojo/dart/packages/mojo_services/lib/mojo/media/rate_control.mojom.dart
index a126702d8181dc671456950a836a8f9d72bd2ee5..d3c69870f309e35d1da73eb8c8be50f94967f825 100644
--- a/mojo/dart/packages/mojo_services/lib/mojo/media/rate_control.mojom.dart
+++ b/mojo/dart/packages/mojo_services/lib/mojo/media/rate_control.mojom.dart
@@ -881,11 +881,14 @@ const int _RateControl_cancelPendingChangesName = 6;
 
 
 class _RateControlServiceDescription 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 RateControl {
@@ -1191,8 +1194,13 @@ class RateControlStub extends bindings.Stub {
 
   int get version => 0;
 
-  service_describer.ServiceDescription get serviceDescription =>
-    new _RateControlServiceDescription();
+  static service_describer.ServiceDescription _cachedServiceDescription;
+  static service_describer.ServiceDescription get serviceDescription {
+    if (_cachedServiceDescription == null) {
+      _cachedServiceDescription = new _RateControlServiceDescription();
+    }
+    return _cachedServiceDescription;
+  }
 }