Expose Service Describer in Dart mojo package

mojo/public/tools/bindings/generators/dart_templates/interface_definition.tmpl

 {%- for method in interface.methods %}
 const int _{{interface|name}}_{{method|name}}Name = {{method.ordinal}};
 {%- endfor %}
 
 {#--- Interface Enums #}
 {%- from "enum_definition.tmpl" import enum_def -%}
 {%- for enum in interface.enums %}
   {{ enum_def(enum, typepkg, package) }}
 {%-  endfor %}
 
 {% if should_gen_mojom_types -%}
 {{ mojom_type_macros.writeMojomTypeDef(interface, typepkg, package) }}
 {%- endif %}
 
 class _{{interface|name}}ServiceDescription implements {{descpkg}}ServiceDescription {
 {%- if should_gen_mojom_types %}
   dynamic getTopLevelInterface([Function responseFactory]) =>
-      _{{interface|mojom_type_identifier|lower_camel}}();
+    responseFactory(_{{interface|mojom_type_identifier|lower_camel}}());
 
   dynamic getTypeDefinition(String typeKey, [Function responseFactory]) =>
-      getAllMojomTypeDefinitions()[typeKey];
+    responseFactory(getAllMojomTypeDefinitions()[typeKey]);
 
   dynamic getAllTypeDefinitions([Function responseFactory]) =>
-      getAllMojomTypeDefinitions();
+    responseFactory(getAllMojomTypeDefinitions());
 {%- else %}
-  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);
 {%- endif %}
 }
 
 abstract class {{interface|name}} {
 {%- if interface.service_name %}
   static const String serviceName = "{{interface.service_name}}";
 {%- else %}
   static const String serviceName = null;
 {%- endif %}
 
@@ skipping 277 matching lines @@
     _impl = d;
   }
 
   String toString() {
     var superString = super.toString();
     return "{{interface|name}}Stub($superString)";
   }
 
   int get version => {{interface.version}};
 
-  {{descpkg}}ServiceDescription get serviceDescription =>
+  static {{descpkg}}ServiceDescription get serviceDescription =>
     new _{{interface|name}}ServiceDescription();
 }