Introduce a BuiltinsConstructStub that sets up new.target and does a [[call]] per ES6 9.3.2

test/mjsunit/regexp.js

 // Copyright 2012 the V8 project authors. All rights reserved.
 // Redistribution and use in source and binary forms, with or without
 // modification, are permitted provided that the following conditions are
 // met:
 //
 //     * Redistributions of source code must retain the above copyright
 //       notice, this list of conditions and the following disclaimer.
 //     * Redistributions in binary form must reproduce the above
 //       copyright notice, this list of conditions and the following
 //       disclaimer in the documentation and/or other materials provided
@@ skipping 257 matching lines @@
 // arguments:
 re = new RegExp();
 // KJS actually shows this as '//'.  Here we match the Firefox behavior (ie,
 // giving a syntactically legal regexp literal).
 assertEquals('/(?:)/', re.toString());
 re = new RegExp(void 0);
 assertEquals('/(?:)/', re.toString());
 re.compile();
 assertEquals('/(?:)/', re.toString());
 re.compile(void 0);
-assertEquals('/undefined/', re.toString());
+assertEquals('/(?:)/', re.toString());
 
 
 // Check for lazy RegExp literal creation
 function lazyLiteral(doit) {
   if (doit) return "".replace(/foo(/gi, "");
   return true;
 }
 
 assertTrue(lazyLiteral(false));
 assertThrows("lazyLiteral(true)");
@@ skipping 426 matching lines @@
 // Test that RegExp.prototype.toString() throws TypeError for
 // incompatible receivers (ES5 section 15.10.6 and 15.10.6.4).
 assertThrows("RegExp.prototype.toString.call(null)", TypeError);
 assertThrows("RegExp.prototype.toString.call(0)", TypeError);
 assertThrows("RegExp.prototype.toString.call('')", TypeError);
 assertThrows("RegExp.prototype.toString.call(false)", TypeError);
 assertThrows("RegExp.prototype.toString.call(true)", TypeError);
 assertThrows("RegExp.prototype.toString.call([])", TypeError);
 assertThrows("RegExp.prototype.toString.call({})", TypeError);
 assertThrows("RegExp.prototype.toString.call(function(){})", TypeError);