Implement `in' for proxies.

src/messages.js

Index: src/messages.js
diff --git a/src/messages.js b/src/messages.js
index b9281070a7670f6d38b1e7e5ddea603d551e9cc2..98f24408689a550d273a3c2fa16bbd4786cd69bb 100644
--- a/src/messages.js
+++ b/src/messages.js
@@ -195,7 +195,8 @@ function FormatMessage(message) {
       non_extensible_proto:         ["%0", " is not extensible"],
       handler_non_object:           ["Proxy.", "%0", " called with non-object as handler"],
       handler_trap_missing:         ["Proxy handler ", "%0", " has no '", "%1", "' trap"],
-      handler_failed:               ["Proxy handler ", "%0", " returned false for '", "%1", "' trap"],
+      handler_returned_false:       ["Proxy handler ", "%0", " returned false for '", "%1", "' trap"],
+      handler_returned_undefined:   ["Proxy handler ", "%0", " returned undefined for '", "%1", "' trap"],
       proxy_prop_not_configurable:  ["Trap ", "%1", " of proxy handler ", "%0", " returned non-configurable descriptor for property ", "%2"],
       proxy_non_object_prop_names:  ["Trap ", "%1", " returned non-object ", "%0"],
       proxy_repeated_prop_name:     ["Trap ", "%1", " returned repeated property name ", "%2"],