[es6] Fix Object built-in subclassing.

test/mjsunit/es6/classes-subclass-builtins.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: --allow-natives-syntax
 
 "use strict";
 
 
 function checkPrototypeChain(object, constructors) {
   var proto = object.__proto__;
   for (var i = 0; i < constructors.length; i++) {
     assertEquals(constructors[i].prototype, proto);
     assertEquals(constructors[i], proto.constructor);
     proto = proto.__proto__;
   }
 }
 
 
 (function() {
+  class A extends Object {
+    constructor(...args) {
+      assertTrue(%IsConstructCall());
+      super(...args);
+      this.a = 42;
+    }
+  }
+
+  var s = new A("foo");
+  assertTrue(s instanceof Object);
+  assertTrue(s instanceof A);
+  assertEquals("object", typeof s);
+  checkPrototypeChain(s, [A, Object]);
+  assertEquals(42, s.a);
+
+  var s1 = new A("bar");
+  assertTrue(%HaveSameMap(s, s1));
+
+
+  var n = new A(153);
+  assertTrue(n instanceof Object);
+  assertTrue(n instanceof A);
+  assertEquals("object", typeof s);
+  checkPrototypeChain(s, [A, Object]);
+  assertEquals(42, n.a);
+
+  var n1 = new A(312);
+  assertTrue(%HaveSameMap(n, n1));
+  assertTrue(%HaveSameMap(n, s));
+
+
+  var b = new A(true);
+  assertTrue(b instanceof Object);
+  assertTrue(b instanceof A);
+  assertEquals("object", typeof s);
+  checkPrototypeChain(s, [A, Object]);
+  assertEquals(42, b.a);
+
+  var b1 = new A(true);
+  assertTrue(%HaveSameMap(b, b1));
+  assertTrue(%HaveSameMap(b, s));
+})();
+
+
+(function() {
   class A extends Function {
     constructor(...args) {
       assertTrue(%IsConstructCall());
       super(...args);
       this.a = 42;
     }
   }
 
   var o = new A("this.foo = 153;");
   assertTrue(o instanceof Object);
@@ skipping 480 matching lines @@
 (function() {
   class A extends null {}
   assertThrows("new A");
 })();
 
 
 (function() {
   class A extends Symbol {}
   assertThrows("new A");
 })();