Use strict mode and auto-throwing instead of an explicit throw.

es6-shim.js

@@ -85,6 +85,10 @@
   var globals = getGlobal();
   var globalIsFinite = globals.isFinite;
   var supportsDescriptors = !!Object.defineProperty && arePropertyDescriptorsSupported();
+  var hasStrictMode = (function () {
+    'use strict';
+    return this === null;
+  }.call(null));
   var startsWithIsCompliant = startsWithRejectsRegex() && startsWithHandlesInfinity;
   var _indexOf = Function.call.bind(String.prototype.indexOf);
   var _toString = Function.call.bind(Object.prototype.toString);
@@ -1137,16 +1141,13 @@
     };
     defineProperties(Object, ObjectShims);
 
-    var assignHasPendingExceptions = supportsDescriptors && (function () {
+    var assignHasPendingExceptions = supportsDescriptors && hasStrictMode && (function () {
+      'use strict';
       // Firefox 37 still has "pending exception" logic in its Object.assign implementation,
       // which is 72% slower than our shim, and Firefox 40's native implementation.
       var thrower = { a: 1, b: 2, c: 3 };
       var keys = Object.keys(thrower);
-      Object.defineProperty(thrower, keys[1], {
-        set: function setMiddle(v) {
-          throw new TypeError('middle key can not be set');
-        }
-      });
+      Object.defineProperty(thrower, keys[1], { configurable: false, writable: false });
       var semaphore = {};
       try {
         Object.assign(thrower, { a: semaphore, b: semaphore, c: semaphore });