Expose Service Describer in Dart mojo package

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

Index: mojo/dart/packages/mojo_services/lib/mojo/media/media_clock.mojom.dart
diff --git a/mojo/dart/packages/mojo_services/lib/mojo/media/media_clock.mojom.dart b/mojo/dart/packages/mojo_services/lib/mojo/media/media_clock.mojom.dart
index e34ae82fcbdabe235c7665b80d53d5404609d38d..9ae421184312cbf6f5e1c42dd57e5a926bf02853 100644
--- a/mojo/dart/packages/mojo_services/lib/mojo/media/media_clock.mojom.dart
+++ b/mojo/dart/packages/mojo_services/lib/mojo/media/media_clock.mojom.dart
@@ -99,11 +99,14 @@ class ClockDisposition extends bindings.MojoEnum {
 
 
 class _ClockServiceDescription 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 Clock {
@@ -259,8 +262,13 @@ class ClockStub extends bindings.Stub {
 
   int get version => 0;
 
-  service_describer.ServiceDescription get serviceDescription =>
-    new _ClockServiceDescription();
+  static service_describer.ServiceDescription _cachedServiceDescription;
+  static service_describer.ServiceDescription get serviceDescription {
+    if (_cachedServiceDescription == null) {
+      _cachedServiceDescription = new _ClockServiceDescription();
+    }
+    return _cachedServiceDescription;
+  }
 }