Expose Service Describer in Dart mojo package

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

Index: mojo/dart/packages/mojo_services/lib/mojo/location_service.mojom.dart
diff --git a/mojo/dart/packages/mojo_services/lib/mojo/location_service.mojom.dart b/mojo/dart/packages/mojo_services/lib/mojo/location_service.mojom.dart
index fbe2c170fdfb3ff7c3764e8afc134cf41c2391c0..fc02e56de8ff71737652c2dd4d58d4d1f25129e1 100644
--- a/mojo/dart/packages/mojo_services/lib/mojo/location_service.mojom.dart
+++ b/mojo/dart/packages/mojo_services/lib/mojo/location_service.mojom.dart
@@ -240,11 +240,14 @@ class LocationServiceUpdatePriority extends bindings.MojoEnum {
 
 
 class _LocationServiceServiceDescription 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 LocationService {
@@ -457,8 +460,13 @@ class LocationServiceStub extends bindings.Stub {
 
   int get version => 0;
 
-  service_describer.ServiceDescription get serviceDescription =>
-    new _LocationServiceServiceDescription();
+  static service_describer.ServiceDescription _cachedServiceDescription;
+  static service_describer.ServiceDescription get serviceDescription {
+    if (_cachedServiceDescription == null) {
+      _cachedServiceDescription = new _LocationServiceServiceDescription();
+    }
+    return _cachedServiceDescription;
+  }
 }