Expose Service Describer in Dart mojo package

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

Index: mojo/dart/packages/mojo_services/lib/mojo/terminal/terminal.mojom.dart
diff --git a/mojo/dart/packages/mojo_services/lib/mojo/terminal/terminal.mojom.dart b/mojo/dart/packages/mojo_services/lib/mojo/terminal/terminal.mojom.dart
index ad668c290eb17479a001a8ccf9f5bb85159c6637..be5faa0a2824afa0220dd31bfe4dce27081f668b 100644
--- a/mojo/dart/packages/mojo_services/lib/mojo/terminal/terminal.mojom.dart
+++ b/mojo/dart/packages/mojo_services/lib/mojo/terminal/terminal.mojom.dart
@@ -726,11 +726,14 @@ const int _Terminal_setSizeName = 3;
 
 
 class _TerminalServiceDescription 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 Terminal {
@@ -1121,8 +1124,13 @@ class TerminalStub extends bindings.Stub {
 
   int get version => 0;
 
-  service_describer.ServiceDescription get serviceDescription =>
-    new _TerminalServiceDescription();
+  static service_describer.ServiceDescription _cachedServiceDescription;
+  static service_describer.ServiceDescription get serviceDescription {
+    if (_cachedServiceDescription == null) {
+      _cachedServiceDescription = new _TerminalServiceDescription();
+    }
+    return _cachedServiceDescription;
+  }
 }