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

test/mjsunit/es6/promises.js

Index: test/mjsunit/es6/promises.js
diff --git a/test/mjsunit/es6/promises.js b/test/mjsunit/es6/promises.js
index 19239b601bc74e10c22565cdcfc8f16687759fdf..405da02b53999493215658c07cc2025ff586ac5c 100644
--- a/test/mjsunit/es6/promises.js
+++ b/test/mjsunit/es6/promises.js
@@ -62,7 +62,15 @@ function clearProp(o, name) {
 
 // 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]
@@ -97,7 +105,6 @@ function assertAsyncDone(iteration) {
   });
 }
 
-
 (function() {
   assertThrows(function() { Promise(function() {}) }, TypeError)
 })();