[proxies] Implement [[PreventExtensions]] and [[IsExtensible]].

test/mjsunit/harmony/proxies-is-extensible.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: --harmony-proxies --harmony-reflect
+
+
+(function () {
+  // No trap.
+
+  var target = {};
+  var handler = {};
+  var proxy = new Proxy(target, handler);
+
+  assertTrue(Reflect.isExtensible(target));
+  assertTrue(Reflect.isExtensible(proxy));
+  assertTrue(Reflect.preventExtensions(proxy));
+  assertFalse(Reflect.isExtensible(target));
+  assertFalse(Reflect.isExtensible(proxy));
+})();
+
+
+(function () {
+  // "Undefined" trap.
+
+  var target = {};
+  var handler = { isExtensible: null };
+  var proxy = new Proxy(target, handler);
+
+  assertTrue(Reflect.isExtensible(target));
+  assertTrue(Reflect.isExtensible(proxy));
+  assertTrue(Reflect.preventExtensions(proxy));
+  assertFalse(Reflect.isExtensible(target));
+  assertFalse(Reflect.isExtensible(proxy));
+})();
+
+
+(function () {
+  // Invalid trap.
+
+  var target = {};
+  var handler = { isExtensible: true };
+  var proxy = new Proxy(target, handler);
+
+  assertThrows(() => {Reflect.isExtensible(proxy)}, TypeError);
+})();
+
+
+(function () {
+  var target = {};
+  var handler = { isExtensible() {return "bla"} };
+  var proxy = new Proxy(target, handler);
+
+  // Trap returns trueish and target is extensible.
+  assertTrue(Reflect.isExtensible(proxy));
+
+  // Trap returns trueish but target is not extensible.
+  Reflect.preventExtensions(target);
+  assertThrows(() => {Reflect.isExtensible(proxy)}, TypeError);
+})();
+
+
+(function () {
+  var target = {};
+  var handler = { isExtensible() {return 0} };
+  var proxy = new Proxy(target, handler);
+
+  // Trap returns falsish but target is extensible.
+  assertThrows(() => {Reflect.isExtensible(proxy)}, TypeError);
+
+  // Trap returns falsish and target is not extensible.
+  Reflect.preventExtensions(target);
+  assertFalse(Reflect.isExtensible(proxy));
+})();