Expose Service Describer in Dart mojo package

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

Index: mojo/dart/packages/mojo_services/lib/mojo/host_resolver.mojom.dart
diff --git a/mojo/dart/packages/mojo_services/lib/mojo/host_resolver.mojom.dart b/mojo/dart/packages/mojo_services/lib/mojo/host_resolver.mojom.dart
index 3f6d8afb497c289eea4bd2ffa138d922e07fc951..8c1786290d424e6b6620acb96b3e8e4ff09c08ed 100644
--- a/mojo/dart/packages/mojo_services/lib/mojo/host_resolver.mojom.dart
+++ b/mojo/dart/packages/mojo_services/lib/mojo/host_resolver.mojom.dart
@@ -219,11 +219,14 @@ const int _HostResolver_getHostAddressesName = 0;
 
 
 class _HostResolverServiceDescription 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 HostResolver {
@@ -438,8 +441,13 @@ class HostResolverStub extends bindings.Stub {
 
   int get version => 0;
 
-  service_describer.ServiceDescription get serviceDescription =>
-    new _HostResolverServiceDescription();
+  static service_describer.ServiceDescription _cachedServiceDescription;
+  static service_describer.ServiceDescription get serviceDescription {
+    if (_cachedServiceDescription == null) {
+      _cachedServiceDescription = new _HostResolverServiceDescription();
+    }
+    return _cachedServiceDescription;
+  }
 }