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

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-concat-spreadable
 
 (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;
+  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 664 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);
 })();