Expose Service Describer in Dart mojo package

mojo/dart/packages/mojo/lib/mojo/service_provider.mojom.dart

Index: mojo/dart/packages/mojo/lib/mojo/service_provider.mojom.dart
diff --git a/mojo/dart/packages/mojo/lib/mojo/service_provider.mojom.dart b/mojo/dart/packages/mojo/lib/mojo/service_provider.mojom.dart
index 3b0f8733d6d07b58e0f0aa2f6fa174748768fce5..133beb513acb129a4d17a0b4da69dcc68831dc27 100644
--- a/mojo/dart/packages/mojo/lib/mojo/service_provider.mojom.dart
+++ b/mojo/dart/packages/mojo/lib/mojo/service_provider.mojom.dart
@@ -104,11 +104,14 @@ const int _ServiceProvider_connectToServiceName = 0;
 
 
 class _ServiceProviderServiceDescription 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 ServiceProvider {
@@ -280,8 +283,13 @@ class ServiceProviderStub extends bindings.Stub {
 
   int get version => 0;
 
-  service_describer.ServiceDescription get serviceDescription =>
-    new _ServiceProviderServiceDescription();
+  static service_describer.ServiceDescription _cachedServiceDescription;
+  static service_describer.ServiceDescription get serviceDescription {
+    if (_cachedServiceDescription == null) {
+      _cachedServiceDescription = new _ServiceProviderServiceDescription();
+    }
+    return _cachedServiceDescription;
+  }
 }