[runtime] Avoid @@isConcatSpreadable lookup for fast path Array.prototype.concat

test/mjsunit/es6/array-concat.js

Index: test/mjsunit/es6/array-concat.js
diff --git a/test/mjsunit/es6/array-concat.js b/test/mjsunit/es6/array-concat.js
index fe320d6858665b67f7524f597968bf7b7831314f..00edfd66c1c52466113f7ef649d40fe209ea9d29 100644
--- a/test/mjsunit/es6/array-concat.js
+++ b/test/mjsunit/es6/array-concat.js
@@ -1,7 +1,6 @@
 // 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.
-
 (function testArrayConcatArity() {
   "use strict";
   assertEquals(1, Array.prototype.concat.length);
@@ -20,6 +19,15 @@
   assertEquals(false, desc.enumerable);
 })();
 
+(function testNonConcatSpreadableArray() {
+  "use strict"
+  var array = [1, 2, 3];
+  assertEquals(array, [].concat(array));
+  assertEquals(array, array.concat([]));
+  array[Symbol.isConcatSpreadable] = false;

[Toon Verwaest, 2016/03/31 08:09:46]

Make sure to add tests for all the missing cases. See the tests added by Dan for
his protectors.

+  assertEquals([[1,2,3]], [].concat(array));
+  assertEquals([[1,2,3]], array.concat([]));
+})();
 
 (function testConcatArrayLike() {
   "use strict";