Expose Service Describer in Dart mojo package

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

Index: mojo/dart/packages/mojo_services/lib/mojo/files/file.mojom.dart
diff --git a/mojo/dart/packages/mojo_services/lib/mojo/files/file.mojom.dart b/mojo/dart/packages/mojo_services/lib/mojo/files/file.mojom.dart
index d8c28ce47d221464e96b16e20b3a4942b0edbb60..28d0c9b260309c61b409d9dc081791ba9848ed2a 100644
--- a/mojo/dart/packages/mojo_services/lib/mojo/files/file.mojom.dart
+++ b/mojo/dart/packages/mojo_services/lib/mojo/files/file.mojom.dart
@@ -2395,11 +2395,14 @@ const int _File_ioctlName = 13;
 
 
 class _FileServiceDescription 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 File {
@@ -3369,8 +3372,13 @@ class FileStub extends bindings.Stub {
 
   int get version => 0;
 
-  service_describer.ServiceDescription get serviceDescription =>
-    new _FileServiceDescription();
+  static service_describer.ServiceDescription _cachedServiceDescription;
+  static service_describer.ServiceDescription get serviceDescription {
+    if (_cachedServiceDescription == null) {
+      _cachedServiceDescription = new _FileServiceDescription();
+    }
+    return _cachedServiceDescription;
+  }
 }