[es6] Fix return checking in derived constructors

src/messages.js

Index: src/messages.js
diff --git a/src/messages.js b/src/messages.js
index eadc880715a01d60bd93e394632a3cd2cdcacbd5..41bc364d6a7fee4d4bae21704ce5dc36d6b7d695 100644
--- a/src/messages.js
+++ b/src/messages.js
@@ -203,6 +203,7 @@ var kMessages = {
   duplicate_proto:               ["Duplicate __proto__ fields are not allowed in object literals"],
   param_after_rest:              ["Rest parameter must be last formal parameter"],
   constructor_noncallable:       ["Class constructors cannot be invoked without 'new'"],
+  derived_constructor_return:    ["Derived constructors may only return object or undefined"],
   array_not_subclassable:        ["Subclassing Arrays is not currently supported."],
   for_in_loop_initializer:       ["for-in loop variable declaration may not have an initializer."],
   for_of_loop_initializer:       ["for-of loop variable declaration may not have an initializer."],