Lock the prototype of internal classes.

src/json.js

 // Copyright 2009 the V8 project authors. All rights reserved.
 // Redistribution and use in source and binary forms, with or without
 // modification, are permitted provided that the following conditions are
 // met:
 //
 //     * Redistributions of source code must retain the above copyright
 //       notice, this list of conditions and the following disclaimer.
 //     * Redistributions in binary form must reproduce the above
 //       copyright notice, this list of conditions and the following
 //       disclaimer in the documentation and/or other materials provided
@@ skipping 319 matching lines @@
       gap = SubString(space, 0, 10);
     } else {
       gap = space;
     }
   } else {
     gap = "";
   }
   return JSONSerialize('', {'': value}, replacer, new InternalArray(), "", gap);
 }
 
-function SetupJSON() {
+function SetUpJSON() {
+  %CheckIsBootstrapping();
   InstallFunctions($JSON, DONT_ENUM, $Array(
     "parse", JSONParse,
     "stringify", JSONStringify
   ));
 }
 
-SetupJSON();
+SetUpJSON()