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

test/mjsunit/harmony/array-concat-object-proto-dict-getter.js

+// Copyright 2016 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.
+
+// Check that @@isConcatSpreadable is checked when set on Object.prototype
+// with a dictionary backing store.
+
+// Force Object.prototype into dictionary backing store by adding many
+// properties.
+for (var i = 0; i < 10*1000; i++) {
+  Object.prototype['generatedProperty'+i] = true;
+}
+
+var array = [1, 2, 3];
+var object = {length: 1, '0': 'a'};
+
+function testConcatDefaults() {
+  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));
+}
+
+testConcatDefaults();
+
+var concatSpreadable = false;
+Object.defineProperty(Object.prototype, Symbol.isConcatSpreadable, {
+  get() { return concatSpreadable },
+  configurable: true
+});
+
+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));
+
+concatSpreadable = true;
+
+assertEquals(array, [].concat(array));
+assertEquals(array, array.concat([]));
+assertEquals([1, 2, 3, 1, 2, 3], array.concat(array));
+assertEquals(['a'], [].concat(object));
+assertEquals([1, 2, 3, 'a'], array.concat(object));
+assertEquals(['a'], Array.prototype.concat.call(object,[]));
+assertEquals(['a', 1, 2, 3], Array.prototype.concat.call(object, array));
+assertEquals(['a', 'a'], Array.prototype.concat.call(object, object));
+
+delete Object.prototype[Symbol.isConcatSpreadable];
+testConcatDefaults();