Fix storing to primitive objects by applying ToObject. This is necessary to ensure that we properly…

test/mjsunit/assign-primitive-property.js

+// Copyright 2014 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.
+
+"use strict";
+
+Object.defineProperty(String.prototype, "x", {
+  value: 10,
+  writable: false
+});
+
+assertThrows(function() { "test".x = 20; }, TypeError);
+
+var value = 0;
+Object.defineProperty(String.prototype, "foo", {
+  set: function (data) { value += data }
+});
+
+"test".foo = 20;
+assertEquals(20, value);