[es6] Implement Date.prototype[@@toPrimitive] as C++ builtin.

test/mjsunit/harmony/to-string.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
 
 assertEquals("1", %ToString(1));
 assertEquals("1", %_ToString(1));
 
 assertEquals("0.5", %ToString(.5));
@@ skipping 30 matching lines @@
   valueOf: function() { return 123 }
 };
 assertEquals("x", %ToString(c));
 assertEquals("x", %_ToString(c));
 
 var d = {
   [Symbol.toPrimitive]: function(hint) { return hint }
 };
 assertEquals("string", %ToString(d));
 assertEquals("string", %_ToString(d));
+
+var e = new Date(0);
+assertEquals(e.toString(), %ToName(e));
+assertEquals(e.toString(), %_ToName(e));