Expose Service Describer in Dart mojo package

mojo/dart/packages/mojo_services/lib/mojo/media/media_source.mojom.dart

 // Copyright 2014 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 library media_source_mojom;
 
 import 'dart:async';
 
 import 'package:mojo/bindings.dart' as bindings;
 import 'package:mojo/core.dart' as core;
@@ skipping 1099 matching lines @@
 const int _MediaSource_getMasterClockName = 2;
 const int _MediaSource_setMasterClockName = 3;
 const int _MediaSource_getProducerName = 4;
 const int _MediaSource_getPullModeProducerName = 5;
 const int _MediaSource_getStatusName = 6;
 const int _MediaSource_prepareName = 7;
 
 
 
 class _MediaSourceServiceDescription 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 MediaSource {
   static const String serviceName = null;
   dynamic getStreams([Function responseFactory = null]);
   dynamic getClockDisposition([Function responseFactory = null]);
   void getMasterClock(Object masterClock);
   void setMasterClock(Object masterClock);
   void getProducer(int streamIndex, Object producer);
   void getPullModeProducer(int streamIndex, Object producer);
@@ skipping 422 matching lines @@
     _impl = d;
   }
 
   String toString() {
     var superString = super.toString();
     return "MediaSourceStub($superString)";
   }
 
   int get version => 0;
 
-  service_describer.ServiceDescription get serviceDescription =>
-    new _MediaSourceServiceDescription();
+  static service_describer.ServiceDescription _cachedServiceDescription;
+  static service_describer.ServiceDescription get serviceDescription {
+    if (_cachedServiceDescription == null) {
+      _cachedServiceDescription = new _MediaSourceServiceDescription();
+    }
+    return _cachedServiceDescription;
+  }
 }