[proxies] Make Object.prototype.isPrototypeOf work with proxies.

test/mjsunit/harmony/proxies-get-prototype-of.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
 
 var target = { target: 1 };
 target.__proto__ = {};
 var handler = { handler: 1 };
 var proxy = new Proxy(target, handler);
@@ skipping 18 matching lines @@
 var proxy2 = new Proxy(proxy, {'handler':1});
 assertSame(Object.getPrototypeOf(proxy2), target_prototype);
 
 // Test with Proxy handler:
 var proxy3_prototype = {'proto3':true};
 var handler_proxy = new Proxy({
   getPrototypeOf: function() { return proxy3_prototype }
 }, {});
 var proxy3 = new Proxy(target, handler_proxy);
 assertSame(Object.getPrototypeOf(proxy3), proxy3_prototype);
+
+
+// Some tests with Object.prototype.isPrototypeOf
+
+(function () {
+  var object = {};
+  var handler = {};
+  var proto = new Proxy({}, handler);
+  object.__proto__ = proto;
+
+  assertTrue(proto.isPrototypeOf(object));
+  assertTrue(Object.prototype.isPrototypeOf.call(proto, object));
+
+  handler.getPrototypeOf = function () { return Object.prototype };
+  assertTrue(proto.isPrototypeOf(object));
+  assertTrue(Object.prototype.isPrototypeOf.call(proto, object));
+  assertTrue(Object.prototype.isPrototypeOf(object));
+  assertFalse(Object.prototype.isPrototypeOf.call(Array.prototype, object));
+  assertFalse(Array.prototype.isPrototypeOf(object));
+
+  handler.getPrototypeOf = function () { return object };
+  assertTrue(Object.prototype.isPrototypeOf.call(proto, object));
+  assertTrue(proto.isPrototypeOf(object));
+  assertTrue(Object.prototype.isPrototypeOf.call(object, object));
+  assertTrue(object.isPrototypeOf(object));
+
+  handler.getPrototypeOf = function () { throw "foo" };
+  assertTrue(proto.isPrototypeOf(object));
+  assertTrue(Object.prototype.isPrototypeOf.call(proto, object));
+  assertThrows(()=> Object.prototype.isPrototypeOf(object));
+  assertThrows(()=> Object.prototype.isPrototypeOf.call(Array.prototype, object));
+  assertThrows(()=> Array.prototype.isPrototypeOf(object));
+})();
+
+(function () {
+  var handler = {};
+  var object = new Proxy({}, handler);
+  var proto = {};
+
+  assertFalse(Object.prototype.isPrototypeOf.call(object, object));
+  assertFalse(Object.prototype.isPrototypeOf.call(proto, object));
+  assertTrue(Object.prototype.isPrototypeOf.call(Object.prototype, object));
+
+  handler.getPrototypeOf = function () { return proto };
+  assertTrue(Object.prototype.isPrototypeOf.call(proto, object));
+  assertFalse(Object.prototype.isPrototypeOf.call({}, object));
+  assertTrue(Object.prototype.isPrototypeOf.call(Object.prototype, object));
+
+  handler.getPrototypeOf = function () { return object };
+  assertTrue(Object.prototype.isPrototypeOf.call(object, object));
+
+  handler.getPrototypeOf = function () { throw "foo" };
+  assertThrows(()=> Object.prototype.isPrototypeOf.call(object, object));
+  assertThrows(()=> Object.prototype.isPrototypeOf(object));
+})();