Expose Service Describer in Dart mojo package

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

Index: mojo/dart/packages/mojo_services/lib/mojo/sharing.mojom.dart
diff --git a/mojo/dart/packages/mojo_services/lib/mojo/sharing.mojom.dart b/mojo/dart/packages/mojo_services/lib/mojo/sharing.mojom.dart
index bd335cb928db9db469666df075001c0a7975de49..c869b7419063cf9ebfeca86810801525519d5226 100644
--- a/mojo/dart/packages/mojo_services/lib/mojo/sharing.mojom.dart
+++ b/mojo/dart/packages/mojo_services/lib/mojo/sharing.mojom.dart
@@ -92,11 +92,14 @@ const int _SharingService_shareTextName = 0;
 
 
 class _SharingServiceServiceDescription 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 SharingService {
@@ -267,8 +270,13 @@ class SharingServiceStub extends bindings.Stub {
 
   int get version => 0;
 
-  service_describer.ServiceDescription get serviceDescription =>
-    new _SharingServiceServiceDescription();
+  static service_describer.ServiceDescription _cachedServiceDescription;
+  static service_describer.ServiceDescription get serviceDescription {
+    if (_cachedServiceDescription == null) {
+      _cachedServiceDescription = new _SharingServiceServiceDescription();
+    }
+    return _cachedServiceDescription;
+  }
 }