Added ES5 15.2.3.2 Object.getPrototypeOf.

src/messages.js

Index: src/messages.js
===================================================================
--- src/messages.js	(revision 3543)
+++ src/messages.js	(working copy)
@@ -178,7 +178,8 @@
       result_not_primitive:         "Result of %0 must be a primitive, was %1",
       invalid_json:                 "String '%0' is not valid JSON",
       circular_structure:           "Converting circular structure to JSON",
-      object_keys_non_object:       "Object.keys called on non-object"
+      object_keys_non_object:       "Object.keys called on non-object",
+      object_get_prototype_non_object: "Object.getPrototypeOf called on non-object"
     };
   }
   var format = kMessages[message.type];