Expose Service Describer in Dart mojo package

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

Index: mojo/dart/packages/mojo_services/lib/mojo/navigation.mojom.dart
diff --git a/mojo/dart/packages/mojo_services/lib/mojo/navigation.mojom.dart b/mojo/dart/packages/mojo_services/lib/mojo/navigation.mojom.dart
index c78183cdcde15705a87a12885666d248f3dc263a..de077dd5fbde4c63030169b9761beaa612ca2211 100644
--- a/mojo/dart/packages/mojo_services/lib/mojo/navigation.mojom.dart
+++ b/mojo/dart/packages/mojo_services/lib/mojo/navigation.mojom.dart
@@ -321,11 +321,14 @@ const int _NavigatorHost_didNavigateLocallyName = 2;
 
 
 class _NavigatorHostServiceDescription 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 NavigatorHost {
@@ -527,8 +530,13 @@ class NavigatorHostStub extends bindings.Stub {
 
   int get version => 0;
 
-  service_describer.ServiceDescription get serviceDescription =>
-    new _NavigatorHostServiceDescription();
+  static service_describer.ServiceDescription _cachedServiceDescription;
+  static service_describer.ServiceDescription get serviceDescription {
+    if (_cachedServiceDescription == null) {
+      _cachedServiceDescription = new _NavigatorHostServiceDescription();
+    }
+    return _cachedServiceDescription;
+  }
 }