Add the concept of a V8 extras exports object

test/cctest/test-extra.js

 // Copyright 2015 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.
 
-(function() {
-  throw new Error("boom");
+(function () {
+    'use strict';
+
+    extrasExports.testExtraShouldReturnFive = function () {
+        return 5;
+    };
 })();