Expose Service Describer in Dart mojo package

mojo/dart/packages/_mojo_for_test_only/lib/test/echo_service.mojom.dart

Index: mojo/dart/packages/_mojo_for_test_only/lib/test/echo_service.mojom.dart
diff --git a/mojo/dart/packages/_mojo_for_test_only/lib/test/echo_service.mojom.dart b/mojo/dart/packages/_mojo_for_test_only/lib/test/echo_service.mojom.dart
index 7e5992fa3da274cd5573f30c02e2ac6bbc56c07a..7742d7a3f24d1ec82b50dcfb099b67e583c2a7be 100644
--- a/mojo/dart/packages/_mojo_for_test_only/lib/test/echo_service.mojom.dart
+++ b/mojo/dart/packages/_mojo_for_test_only/lib/test/echo_service.mojom.dart
@@ -480,13 +480,13 @@ mojom_types.MojomInterface _echoServiceEchoService() {
 
 class _EchoServiceServiceDescription implements service_describer.ServiceDescription {
   dynamic getTopLevelInterface([Function responseFactory]) =>
-      _echoServiceEchoService();
+    responseFactory(_echoServiceEchoService());
 
   dynamic getTypeDefinition(String typeKey, [Function responseFactory]) =>
-      getAllMojomTypeDefinitions()[typeKey];
+    responseFactory(getAllMojomTypeDefinitions()[typeKey]);
 
   dynamic getAllTypeDefinitions([Function responseFactory]) =>
-      getAllMojomTypeDefinitions();
+    responseFactory(getAllMojomTypeDefinitions());
 }
 
 abstract class EchoService {
@@ -771,8 +771,13 @@ class EchoServiceStub extends bindings.Stub {
 
   int get version => 0;
 
-  service_describer.ServiceDescription get serviceDescription =>
-    new _EchoServiceServiceDescription();
+  static service_describer.ServiceDescription _cachedServiceDescription;
+  static service_describer.ServiceDescription get serviceDescription {
+    if (_cachedServiceDescription == null) {
+      _cachedServiceDescription = new _EchoServiceServiceDescription();
+    }
+    return _cachedServiceDescription;
+  }
 }