Expose Service Describer in Dart mojo package

mojo/dart/packages/mojo_services/lib/http_server/http_server_factory.mojom.dart

Index: mojo/dart/packages/mojo_services/lib/http_server/http_server_factory.mojom.dart
diff --git a/mojo/dart/packages/mojo_services/lib/http_server/http_server_factory.mojom.dart b/mojo/dart/packages/mojo_services/lib/http_server/http_server_factory.mojom.dart
index 04c279ab63339fb5b049ba2d5d26f238199ce232..9b01d78c950f3b5cf2b12becbc9dbdf4f1c9bfe9 100644
--- a/mojo/dart/packages/mojo_services/lib/http_server/http_server_factory.mojom.dart
+++ b/mojo/dart/packages/mojo_services/lib/http_server/http_server_factory.mojom.dart
@@ -107,11 +107,14 @@ const int _HttpServerFactory_createHttpServerName = 0;
 
 
 class _HttpServerFactoryServiceDescription 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 HttpServerFactory {
@@ -283,8 +286,13 @@ class HttpServerFactoryStub extends bindings.Stub {
 
   int get version => 0;
 
-  service_describer.ServiceDescription get serviceDescription =>
-    new _HttpServerFactoryServiceDescription();
+  static service_describer.ServiceDescription _cachedServiceDescription;
+  static service_describer.ServiceDescription get serviceDescription {
+    if (_cachedServiceDescription == null) {
+      _cachedServiceDescription = new _HttpServerFactoryServiceDescription();
+    }
+    return _cachedServiceDescription;
+  }
 }