Add remaining @@toStringTag symbols to builtins

test/mjsunit/es6/string-iterator.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: --harmony-tostring
 
 function TestStringPrototypeIterator() {
   assertTrue(String.prototype.hasOwnProperty(Symbol.iterator));
   assertFalse("".hasOwnProperty(Symbol.iterator));
   assertFalse("".propertyIsEnumerable(Symbol.iterator));
 }
 TestStringPrototypeIterator();
 
 
 function assertIteratorResult(value, done, result) {
@@ skipping 37 matching lines @@
 
 
 function TestStringIteratorPrototype() {
   var iterator = ""[Symbol.iterator]();
   var StringIteratorPrototype = iterator.__proto__;
   assertFalse(StringIteratorPrototype.hasOwnProperty('constructor'));
   assertEquals(StringIteratorPrototype.__proto__, Object.prototype);
   assertArrayEquals(['next'],
       Object.getOwnPropertyNames(StringIteratorPrototype));
   assertEquals('[object String Iterator]', "" + iterator);
+  assertEquals("String Iterator", StringIteratorPrototype[Symbol.toStringTag]);
+  var desc = Object.getOwnPropertyDescriptor(
+      StringIteratorPrototype, Symbol.toStringTag);
+  assertTrue(desc.configurable);
+  assertFalse(desc.writable);
+  assertEquals("String Iterator", desc.value);
 }
 TestStringIteratorPrototype();
 
 
 function TestForOf() {
   var lo = "\uD834";
   var hi = "\uDF06";
   var pair = lo + hi;
   var string = "abc" + pair + "def" + lo + pair + hi + lo;
   var expected = ['a', 'b', 'c', pair, 'd', 'e', 'f', lo, pair, hi, lo];
 
   var i = 0;
   for (var char of string) {
     assertEquals(expected[i++], char);
   }
 
   assertEquals(expected.length, i);
 }
 TestForOf();
 
 
 function TestNonOwnSlots() {
   var iterator = ""[Symbol.iterator]();
   var object = {__proto__: iterator};
 
   assertThrows(function() { object.next(); }, TypeError);
 }
 TestNonOwnSlots();