Expose Service Describer in Dart mojo package

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

Index: mojo/dart/packages/mojo_services/lib/mojo/content_handler.mojom.dart
diff --git a/mojo/dart/packages/mojo_services/lib/mojo/content_handler.mojom.dart b/mojo/dart/packages/mojo_services/lib/mojo/content_handler.mojom.dart
index c07ac73553da26076aa658a3c7c80f160f9c79ca..d6424afaf039b1f947a30853c57b48b62138f2be 100644
--- a/mojo/dart/packages/mojo_services/lib/mojo/content_handler.mojom.dart
+++ b/mojo/dart/packages/mojo_services/lib/mojo/content_handler.mojom.dart
@@ -107,11 +107,14 @@ const int _ContentHandler_startApplicationName = 0;
 
 
 class _ContentHandlerServiceDescription 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 ContentHandler {
@@ -283,8 +286,13 @@ class ContentHandlerStub extends bindings.Stub {
 
   int get version => 0;
 
-  service_describer.ServiceDescription get serviceDescription =>
-    new _ContentHandlerServiceDescription();
+  static service_describer.ServiceDescription _cachedServiceDescription;
+  static service_describer.ServiceDescription get serviceDescription {
+    if (_cachedServiceDescription == null) {
+      _cachedServiceDescription = new _ContentHandlerServiceDescription();
+    }
+    return _cachedServiceDescription;
+  }
 }