Fix ArrayConcat for JSValues/JSFunctions/JSRegExps with @@isConcatSpreadable

test/mjsunit/harmony/array-concat.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-arrays --harmony-classes
 
 (function testArrayConcatArity() {
   "use strict";
   assertEquals(1, Array.prototype.concat.length);
 })();
@@ skipping 286 matching lines @@
 
 
 (function testConcatHoleySloppyArguments() {
   var args = (function(a) { return arguments; })(1,2,3);
   delete args[1];
   args[Symbol.isConcatSpreadable] = true;
   assertEquals([1, void 0, 3, 1, void 0, 3], [].concat(args, args));
 })();
 
 
+(function testConcatSpreadableStringPrototype() {
+  var str1 = new String("yuck\uD83D\uDCA9")
+  // String wrapper objects are not concat-spreadable by default
+  assertEquals([str1], [].concat(str1));
+
+  String.prototype[Symbol.isConcatSpreadable] = true;
+  // String wrapper objects may be concat-spreadable
+  assertEquals(["y", "u", "c", "k", "\uD83D", "\uDCA9"],
+               [].concat(new String("yuck\uD83D\uDCA9")));
+
+  // String values are never concat-spreadable
+  assertEquals(["yuck\uD83D\uDCA9"], [].concat("yuck\uD83D\uDCA9"));
+  delete String.prototype[Symbol.isConcatSpreadable];
+})();
+
+
 // ES5 tests
 (function testArrayConcatES5() {
   "use strict";
   var poses;
   var pos;
 
   poses = [140, 4000000000];
   while (pos = poses.shift()) {
     var a = new Array(pos);
     var array_proto = [];
@@ skipping 191 matching lines @@
   arr3.length = 10000;
   for (var i = 0; i < 100; i++) {
     Object.defineProperty(arr3, i * i, {get: mkGetter(i)});
     expectedTrace[i] = i;
     expectedTrace[100 + i] = i;
   }
   var r4 = [0].concat(arr3, arr3);
   assertEquals(1 + arr3.length * 2, r4.length);
   assertEquals(expectedTrace, trace);
 })();