Expose Service Describer in Dart mojo package

mojo/dart/packages/mojo_services/lib/mojo/media/audio_track.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 audio_track_mojom;
 
 import 'dart:async';
 
 import 'package:mojo/bindings.dart' as bindings;
 import 'package:mojo/core.dart' as core;
@@ skipping 583 matching lines @@
 
 
 const int _AudioTrack_describeName = 0;
 const int _AudioTrack_configureName = 1;
 const int _AudioTrack_getRateControlName = 2;
 const int _AudioTrack_setGainName = 3;
 
 
 
 class _AudioTrackServiceDescription 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 AudioTrack {
   static const String serviceName = null;
   dynamic describe([Function responseFactory = null]);
   void configure(AudioTrackConfiguration configuration, Object pipe);
   void getRateControl(Object rateControl);
   void setGain(double dbGain);
   static const double kMutedGain = -160.0;
   static const double kMaxGain = 20.0;
@@ skipping 239 matching lines @@
     _impl = d;
   }
 
   String toString() {
     var superString = super.toString();
     return "AudioTrackStub($superString)";
   }
 
   int get version => 0;
 
-  service_describer.ServiceDescription get serviceDescription =>
-    new _AudioTrackServiceDescription();
+  static service_describer.ServiceDescription _cachedServiceDescription;
+  static service_describer.ServiceDescription get serviceDescription {
+    if (_cachedServiceDescription == null) {
+      _cachedServiceDescription = new _AudioTrackServiceDescription();
+    }
+    return _cachedServiceDescription;
+  }
 }