Expose Service Describer in Dart mojo package

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

Index: mojo/dart/packages/mojo_services/lib/mojo/clipboard.mojom.dart
diff --git a/mojo/dart/packages/mojo_services/lib/mojo/clipboard.mojom.dart b/mojo/dart/packages/mojo_services/lib/mojo/clipboard.mojom.dart
index 3efb1695e35dd52f363bcb1cfaa154efe5447de4..075d4751b159413c70b3c5c43289d07265fbfe2e 100644
--- a/mojo/dart/packages/mojo_services/lib/mojo/clipboard.mojom.dart
+++ b/mojo/dart/packages/mojo_services/lib/mojo/clipboard.mojom.dart
@@ -713,11 +713,14 @@ class ClipboardType extends bindings.MojoEnum {
 
 
 class _ClipboardServiceDescription 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 Clipboard {
@@ -1064,8 +1067,13 @@ class ClipboardStub extends bindings.Stub {
 
   int get version => 0;
 
-  service_describer.ServiceDescription get serviceDescription =>
-    new _ClipboardServiceDescription();
+  static service_describer.ServiceDescription _cachedServiceDescription;
+  static service_describer.ServiceDescription get serviceDescription {
+    if (_cachedServiceDescription == null) {
+      _cachedServiceDescription = new _ClipboardServiceDescription();
+    }
+    return _cachedServiceDescription;
+  }
 }