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

test/mjsunit/harmony/array-concat-array-proto.js

+// Copyright 2014 the V8 project authors. All rights reserved.

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

http://s2.quickmeme.com/img/98/98b39b21ff008afaa5cba1e79fc611b2ddb64b3f44f57e...

+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+// Check that @@isConcatSpreadable is checked when set on Object.prototype
+
+"use strict"
+
+var array = [1, 2, 3];
+var object = {length: 1, '0': 'a'};
+
+assertEquals(array, [].concat(array));
+assertEquals(array, array.concat([]));
+assertEquals([1, 2, 3, 1, 2, 3], array.concat(array));
+assertEquals([object], [].concat(object));
+assertEquals([1, 2, 3, object], array.concat(object));
+assertEquals([object], Array.prototype.concat.call(object,[]));
+assertEquals([object, 1, 2, 3], Array.prototype.concat.call(object, array));
+assertEquals([object, object], Array.prototype.concat.call(object, object));
+
+Array.prototype[Symbol.isConcatSpreadable] = false;
+
+assertEquals([[], array], [].concat(array));
+assertEquals([array, []], array.concat([]));
+assertEquals([array, array], array.concat(array));
+assertEquals([[], object], [].concat(object));
+assertEquals([array, object], array.concat(object));
+assertEquals([object, []], Array.prototype.concat.call(object,[]));
+assertEquals([object, array], Array.prototype.concat.call(object, array));
+assertEquals([object, object], Array.prototype.concat.call(object, object));
+
+Array.prototype[Symbol.isConcatSpreadable] = true;
+
+assertEquals(array, [].concat(array));
+assertEquals(array, array.concat([]));
+assertEquals([1, 2, 3, 1, 2, 3], array.concat(array));
+assertEquals([object], [].concat(object));
+assertEquals([1, 2, 3, object], array.concat(object));
+assertEquals([object], Array.prototype.concat.call(object,[]));
+assertEquals([object, 1, 2, 3], Array.prototype.concat.call(object, array));
+assertEquals([object, object], Array.prototype.concat.call(object, object));