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

test/mjsunit/es6/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.
-
 (function testArrayConcatArity() {
   "use strict";
   assertEquals(1, Array.prototype.concat.length);
 })();
 
 
 (function testArrayConcatNoPrototype() {
   "use strict";
   assertEquals(void 0, Array.prototype.concat.prototype);
 })();
 
 
 (function testArrayConcatDescriptor() {
   "use strict";
   var desc = Object.getOwnPropertyDescriptor(Array.prototype, 'concat');
   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";
   var obj = {
     "length": 6,
     "1": "A",
     "3": "B",
     "5": "C"
   };
   obj[Symbol.isConcatSpreadable] = true;
@@ skipping 824 matching lines @@
     if (key === "length") return Symbol();
     if (key === "2") return "baz";
     if (key === "3") return "bar";
   };
   var target = [];
   var obj = new Proxy(target, {get: getTrap, has: () => true});
 
   assertThrows(() => [].concat(obj), TypeError);
   assertThrows(() => Array.prototype.concat.apply(obj), TypeError);
 })();