Expose Service Describer in Dart mojo package

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

Index: mojo/dart/packages/mojo_services/lib/mojo/network_service.mojom.dart
diff --git a/mojo/dart/packages/mojo_services/lib/mojo/network_service.mojom.dart b/mojo/dart/packages/mojo_services/lib/mojo/network_service.mojom.dart
index 2c42b2c7672f84c51941985457ecfbc8eedff9ac..9396e8ef9bf185c7114e309e5d9aec0d6e186ef2 100644
--- a/mojo/dart/packages/mojo_services/lib/mojo/network_service.mojom.dart
+++ b/mojo/dart/packages/mojo_services/lib/mojo/network_service.mojom.dart
@@ -1032,11 +1032,14 @@ const int _NetworkService_createHostResolverName = 8;
 
 
 class _NetworkServiceServiceDescription 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 NetworkService {
@@ -1461,8 +1464,13 @@ class NetworkServiceStub extends bindings.Stub {
 
   int get version => 0;
 
-  service_describer.ServiceDescription get serviceDescription =>
-    new _NetworkServiceServiceDescription();
+  static service_describer.ServiceDescription _cachedServiceDescription;
+  static service_describer.ServiceDescription get serviceDescription {
+    if (_cachedServiceDescription == null) {
+      _cachedServiceDescription = new _NetworkServiceServiceDescription();
+    }
+    return _cachedServiceDescription;
+  }
 }