Expose Service Describer in Dart mojo package

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

Index: mojo/dart/packages/mojo_services/lib/authentication/authentication.mojom.dart
diff --git a/mojo/dart/packages/mojo_services/lib/authentication/authentication.mojom.dart b/mojo/dart/packages/mojo_services/lib/authentication/authentication.mojom.dart
index 83a02e11423f6643d4a93db29224c06142b0cb02..4861bf2eb2050cf27568e37980940abed52765d0 100644
--- a/mojo/dart/packages/mojo_services/lib/authentication/authentication.mojom.dart
+++ b/mojo/dart/packages/mojo_services/lib/authentication/authentication.mojom.dart
@@ -447,11 +447,14 @@ const int _AuthenticationService_clearOAuth2TokenName = 2;
 
 
 class _AuthenticationServiceServiceDescription 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 AuthenticationService {
@@ -739,8 +742,13 @@ class AuthenticationServiceStub extends bindings.Stub {
 
   int get version => 0;
 
-  service_describer.ServiceDescription get serviceDescription =>
-    new _AuthenticationServiceServiceDescription();
+  static service_describer.ServiceDescription _cachedServiceDescription;
+  static service_describer.ServiceDescription get serviceDescription {
+    if (_cachedServiceDescription == null) {
+      _cachedServiceDescription = new _AuthenticationServiceServiceDescription();
+    }
+    return _cachedServiceDescription;
+  }
 }