Implemented Object.keys.

src/messages.js

Index: src/messages.js
diff --git a/src/messages.js b/src/messages.js
index 255e54409971da928676b115debd784fa38ecefa..651306713547164e10db3dea5d6c8f9b6107d528 100644
--- a/src/messages.js
+++ b/src/messages.js
@@ -167,7 +167,8 @@ function FormatMessage(message) {
       no_input_to_regexp:           "No input to %0",
       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"
+      circular_structure:           "Converting circular structure to JSON",
+      object_keys_non_object:       "Object.keys called on non-object"
     };
   }
   var format = kMessages[message.type];