[es6] Fix Object built-in subclassing.

test/mjsunit/es6/super.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.
 
 // Flags: --allow-natives-syntax
 // Flags: --harmony-destructuring --harmony-rest-parameters --harmony-sloppy
 
 (function TestSuperNamedLoads() {
   function Base() { }
   function fBase() { }
@@ skipping 1960 matching lines @@
   new F();
   assertEquals(1, calls);
 }());
 
 
 (function TestExtendsObject() {
   'use strict';
   class F extends Object { }
   var f = new F(42);
 
-  // TODO(dslomov,arv): Fix this. BUG=v8:3886.
-  assertInstanceof(f, Number);
+  assertInstanceof(f, F);
+  assertInstanceof(f, Object);
 }());
 
 
 (function TestSuperCallErrorCases() {
   'use strict';
   class T extends Object {
     constructor() {
       super();
     }
   }
@@ skipping 214 matching lines @@
   }
   class Derived extends Base {
     constructor(x) {
       let r = (() => super(...[x]))();
       assertEquals(this, r);
     }
   }
   let d = new Derived(42);
   assertSame(42, d.x);
 })();