Expose Service Describer in Dart mojo package

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

Index: mojo/dart/packages/mojo_services/lib/mojo/url_loader.mojom.dart
diff --git a/mojo/dart/packages/mojo_services/lib/mojo/url_loader.mojom.dart b/mojo/dart/packages/mojo_services/lib/mojo/url_loader.mojom.dart
index 15da776a5eb478ffd0c605fd7c89511ab26ad15c..def335154405a9d5bdd7fac4cedb7d376a620cdc 100644
--- a/mojo/dart/packages/mojo_services/lib/mojo/url_loader.mojom.dart
+++ b/mojo/dart/packages/mojo_services/lib/mojo/url_loader.mojom.dart
@@ -529,11 +529,14 @@ const int _UrlLoader_queryStatusName = 2;
 
 
 class _UrlLoaderServiceDescription 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 UrlLoader {
@@ -858,8 +861,13 @@ class UrlLoaderStub extends bindings.Stub {
 
   int get version => 0;
 
-  service_describer.ServiceDescription get serviceDescription =>
-    new _UrlLoaderServiceDescription();
+  static service_describer.ServiceDescription _cachedServiceDescription;
+  static service_describer.ServiceDescription get serviceDescription {
+    if (_cachedServiceDescription == null) {
+      _cachedServiceDescription = new _UrlLoaderServiceDescription();
+    }
+    return _cachedServiceDescription;
+  }
 }