Fix issue with __proto__ when using ES6 object literals

test/mjsunit/object-literal-multiple-proto-fields.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.
+
+var p1 = {};
+var p2 = {};
+var p3 = {};
+var x = 0;
+var y = 0;
+var z = 0;
+var o = {
+  __proto__: (x++, p1),
+  __proto__: (y++, p2),
+  __proto__: (z++, p3)
+};
+assertEquals(1, x);
+assertEquals(1, y);
+assertEquals(1, z);
+assertEquals(Object.getPrototypeOf(o), p3);