[foozzie] Improve mocking stack traces

tools/foozzie/v8_mock.js

 // Copyright 2016 the V8 project authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 // This is intended for permanent JS behavior changes for mocking out
 // non-deterministic behavior. For temporary suppressions, please refer to
 // v8_suppressions.js.
 // This file is loaded before each correctness test cases and won't get
 // minimized.
 
@@ skipping 37 matching lines @@
     },
   }
 
   Date = new Proxy(Date, handler);
 })();
 
 // Mock stack traces.
 Error.prepareStackTrace = function (error, structuredStackTrace) {
   return "";
 };
-Object.freeze(Error)
+Object.defineProperty(
+    Error, 'prepareStackTrace', { configurable: false, writable: false });
 
 // Mock buffer access in float typed arrays because of varying NaN patterns.
 // Note, for now we just use noop forwarding proxies, because they already
 // turn off optimizations.
 function __MockTypedArray(arrayType) {
   var array_creation_handler = {
     construct: function(target, args) {
       return new Proxy(new arrayType(args), {});
     },
   };
@@ skipping 19 matching lines @@
     }
     this.getMessage = function(){
       index = (index + 1) % 10;
       return workerMessages[index];
     }
     this.postMessage = function(msg){
       __PrettyPrint(msg);
     }
   };
 })();