Expose Service Describer in Dart mojo package

mojo/dart/packages/mojo_services/lib/mojo/gfx/composition/compositor.mojom.dart

Index: mojo/dart/packages/mojo_services/lib/mojo/gfx/composition/compositor.mojom.dart
diff --git a/mojo/dart/packages/mojo_services/lib/mojo/gfx/composition/compositor.mojom.dart b/mojo/dart/packages/mojo_services/lib/mojo/gfx/composition/compositor.mojom.dart
index 7428c77ca820bb9a1dfd1351000bd4fb6fd4975a..091c1690a1f00ce12ce645c539ebdd2a697a1865 100644
--- a/mojo/dart/packages/mojo_services/lib/mojo/gfx/composition/compositor.mojom.dart
+++ b/mojo/dart/packages/mojo_services/lib/mojo/gfx/composition/compositor.mojom.dart
@@ -285,11 +285,14 @@ const int _Compositor_createRendererName = 1;
 
 
 class _CompositorServiceDescription 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 Compositor {
@@ -520,8 +523,13 @@ class CompositorStub extends bindings.Stub {
 
   int get version => 0;
 
-  service_describer.ServiceDescription get serviceDescription =>
-    new _CompositorServiceDescription();
+  static service_describer.ServiceDescription _cachedServiceDescription;
+  static service_describer.ServiceDescription get serviceDescription {
+    if (_cachedServiceDescription == null) {
+      _cachedServiceDescription = new _CompositorServiceDescription();
+    }
+    return _cachedServiceDescription;
+  }
 }