Don’t smash globals used by the test helper itself.

test/mjsunit/es6/promises.js

 // Copyright 2013 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 44 matching lines @@
   var poisoned = {caller: 0, callee: 0, arguments: 0}
   try {
     var x = o[name]
     o[name] = undefined
     clear(x)
   } catch(e) {} // assertTrue(name in poisoned) }
 }
 
 // Find intrinsics and null them out.
 var globals = Object.getOwnPropertyNames(this)
-var whitelist = {Promise: true, TypeError: true}
+var whitelist = {
+  Promise: true,
+  TypeError: true,
+  String: true,
+  JSON: true,
+  Error: true,
+  MjsUnitAssertionError: true
+};
+
 for (var i in globals) {
   var name = globals[i]
   if (name in whitelist || name[0] === name[0].toLowerCase()) delete globals[i]
 }
 for (var i in globals) {
   if (globals[i]) clearProp(this, globals[i])
 }
 
 
 var asyncAssertsExpected = 0;
@@ skipping 14 matching lines @@
   %EnqueueMicrotask(function() {
     if (asyncAssertsExpected === 0)
       assertAsync(true, "all")
     else if (iteration > 10)  // Shouldn't take more.
       assertAsync(false, "all")
     else
       assertAsyncDone(iteration + 1)
   });
 }
 
-
 (function() {
   assertThrows(function() { Promise(function() {}) }, TypeError)
 })();
 
 (function() {
   assertTrue(new Promise(function() {}) instanceof Promise)
 })();
 
 (function() {
   assertThrows(function() { new Promise(5) }, TypeError)
@@ skipping 912 matching lines @@
   Promise.all([11, Promise.accept(12), 13, MyPromise.accept(14), 15, 16])
   assertTrue(log === "nx14n", "subclass/all/arg")
 
   log = ""
   MyPromise.all([21, Promise.accept(22), 23, MyPromise.accept(24), 25, 26])
   assertTrue(log === "nx24nnx21nnx23nnnx25nnx26n", "subclass/all/self")
 })();
 
 
 assertAsyncDone()