Don't allow seal or element property definition on external arrays.

src/messages.js

Index: src/messages.js
diff --git a/src/messages.js b/src/messages.js
index 3c85d9416af035f37c302e4e7df5c29fee9c234b..4adcb307462fa83af4319cb18d19fb51aa22ea25 100644
--- a/src/messages.js
+++ b/src/messages.js
@@ -239,6 +239,8 @@ function FormatMessage(message) {
       "strict_poison_pill",           ["'caller', 'callee', and 'arguments' properties may not be accessed on strict mode functions or the arguments objects for calls to them"],
       "strict_caller",                ["Illegal access to a strict mode caller function."],
       "unprotected_let",              ["Illegal let declaration in unprotected statement context."],
+      "cant_prevent_ext_external_array_elements", ["Cannot prevent extension of an object with external array elements"],
+      "redef_external_array_element", ["Cannot redefine a property of an object with external array elements"],
     ];
     var messages = { __proto__ : null };
     var desc = new PropertyDescriptor();