Ensure object literal element boilerplates aren't modified.

test/mjsunit/regress/regress-466993.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.
+
+// Flags: --allow-natives-syntax
+
+var test = function() {
+  var a = {"1": false, "2": false, "3": false, "4": false};
+  assertEquals(false, a[1]);
+  a[1] = true;
+};
+test();
+test();
+test();
+%OptimizeFunctionOnNextCall(test);
+test();
+test();
+test();