Fix return value of push() and unshift() on Array.prototype.

test/mjsunit/regress/regress-builtinbust-3.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.
+
+function produce_object() {
+  var real_length = 1;
+  function set_length() { real_length = "boom"; }
+  function get_length() { return real_length; }
+  var o = { __proto__:Array.prototype , 0:"x" };
+  Object.defineProperty(o, "length", { set:set_length, get:get_length })
+  return o;
+}
+
+assertEquals(2, produce_object().push("y"));
+assertEquals(2, produce_object().unshift("y"));