[es6] Fix Object built-in subclassing.

test/mjsunit/es6/super.js

Index: test/mjsunit/es6/super.js
diff --git a/test/mjsunit/es6/super.js b/test/mjsunit/es6/super.js
index 4301f69e43c67ad12521236cd46afdb021dca6b5..f93b259fd2a951fecdc02b267ee1939bac0aa21f 100644
--- a/test/mjsunit/es6/super.js
+++ b/test/mjsunit/es6/super.js
@@ -1978,8 +1978,8 @@ TestKeyedSetterCreatingOwnPropertiesNonConfigurable(42, 43, 44);
   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);
 }());