Expose Service Describer in Dart mojo package

mojo/dart/packages/mojo_services/lib/vsync/vsync.mojom.dart

Index: mojo/dart/packages/mojo_services/lib/vsync/vsync.mojom.dart
diff --git a/mojo/dart/packages/mojo_services/lib/vsync/vsync.mojom.dart b/mojo/dart/packages/mojo_services/lib/vsync/vsync.mojom.dart
index 9398004b344734f78bc1db906e629ecd150512d3..1e37acc080b4dac626f454b5229f6c26976c1063 100644
--- a/mojo/dart/packages/mojo_services/lib/vsync/vsync.mojom.dart
+++ b/mojo/dart/packages/mojo_services/lib/vsync/vsync.mojom.dart
@@ -152,11 +152,14 @@ const int _VSyncProvider_awaitVSyncName = 0;
 
 
 class _VSyncProviderServiceDescription 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 VSyncProvider {
@@ -368,8 +371,13 @@ class VSyncProviderStub extends bindings.Stub {
 
   int get version => 0;
 
-  service_describer.ServiceDescription get serviceDescription =>
-    new _VSyncProviderServiceDescription();
+  static service_describer.ServiceDescription _cachedServiceDescription;
+  static service_describer.ServiceDescription get serviceDescription {
+    if (_cachedServiceDescription == null) {
+      _cachedServiceDescription = new _VSyncProviderServiceDescription();
+    }
+    return _cachedServiceDescription;
+  }
 }