Introduce InterfaceEndpointClient(IEC), InterfaceEndpointHandle and

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

Index: mojo/public/tools/bindings/generators/js_templates/interface_definition.tmpl
diff --git a/mojo/public/tools/bindings/generators/js_templates/interface_definition.tmpl b/mojo/public/tools/bindings/generators/js_templates/interface_definition.tmpl
index 54e2d4ebcf187e68193c57d78a565f0a1e06c97d..11e319c1f7c819840a6f2a323f1021f2acacce8f 100644
--- a/mojo/public/tools/bindings/generators/js_templates/interface_definition.tmpl
+++ b/mojo/public/tools/bindings/generators/js_templates/interface_definition.tmpl
@@ -93,7 +93,7 @@
 {%- if method.response_parameters != None %}
     case k{{interface.name}}_{{method.name}}_Name:
       var params = reader.decodeStruct({{interface.name}}_{{method.name}}_Params);
-      return this.{{method.name|stylize_method}}(
+      this.{{method.name|stylize_method}}(
 {%- for parameter in method.parameters -%}
 params.{{parameter.name}}{% if not loop.last %}, {% endif -%}
 {%- endfor %}).then(function(response) {
@@ -111,10 +111,11 @@ params.{{parameter.name}}{% if not loop.last %}, {% endif -%}
         var message = builder.finish();
         responder.accept(message);
       });
+      return true;
 {%- endif %}
 {%- endfor %}
     default:
-      return Promise.reject(Error("Unhandled message: " + reader.messageName));
+      return false;
     }
   };