Expose Service Describer in Dart mojo package

mojo/dart/packages/mojo_services/lib/mojo/files/directory.mojom.dart

Index: mojo/dart/packages/mojo_services/lib/mojo/files/directory.mojom.dart
diff --git a/mojo/dart/packages/mojo_services/lib/mojo/files/directory.mojom.dart b/mojo/dart/packages/mojo_services/lib/mojo/files/directory.mojom.dart
index 14d79b374792b0e0a175a49d6343aff1629c8d7f..da3474e4533929d5564904ebad8c88114fb6ef85 100644
--- a/mojo/dart/packages/mojo_services/lib/mojo/files/directory.mojom.dart
+++ b/mojo/dart/packages/mojo_services/lib/mojo/files/directory.mojom.dart
@@ -1203,11 +1203,14 @@ const int _Directory_deleteName = 6;
 
 
 class _DirectoryServiceDescription 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 Directory {
@@ -1769,8 +1772,13 @@ class DirectoryStub extends bindings.Stub {
 
   int get version => 0;
 
-  service_describer.ServiceDescription get serviceDescription =>
-    new _DirectoryServiceDescription();
+  static service_describer.ServiceDescription _cachedServiceDescription;
+  static service_describer.ServiceDescription get serviceDescription {
+    if (_cachedServiceDescription == null) {
+      _cachedServiceDescription = new _DirectoryServiceDescription();
+    }
+    return _cachedServiceDescription;
+  }
 }