Expose Service Describer in Dart mojo package

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

Index: mojo/dart/packages/mojo_services/lib/mojo/native_viewport_event_dispatcher.mojom.dart
diff --git a/mojo/dart/packages/mojo_services/lib/mojo/native_viewport_event_dispatcher.mojom.dart b/mojo/dart/packages/mojo_services/lib/mojo/native_viewport_event_dispatcher.mojom.dart
index 2a6dbdeb3b0e2a0980e332b004ac16ff3a2473d6..13c15d65d2c76597f34702f00f571c7114d23cb6 100644
--- a/mojo/dart/packages/mojo_services/lib/mojo/native_viewport_event_dispatcher.mojom.dart
+++ b/mojo/dart/packages/mojo_services/lib/mojo/native_viewport_event_dispatcher.mojom.dart
@@ -154,11 +154,14 @@ const int _NativeViewportEventDispatcher_onEventName = 0;
 
 
 class _NativeViewportEventDispatcherServiceDescription 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 NativeViewportEventDispatcher {
@@ -370,8 +373,13 @@ class NativeViewportEventDispatcherStub extends bindings.Stub {
 
   int get version => 0;
 
-  service_describer.ServiceDescription get serviceDescription =>
-    new _NativeViewportEventDispatcherServiceDescription();
+  static service_describer.ServiceDescription _cachedServiceDescription;
+  static service_describer.ServiceDescription get serviceDescription {
+    if (_cachedServiceDescription == null) {
+      _cachedServiceDescription = new _NativeViewportEventDispatcherServiceDescription();
+    }
+    return _cachedServiceDescription;
+  }
 }