Optimize @@species based on a global 'protector' cell

test/mjsunit/harmony/array-species-proto.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.
+
+// Flags: --harmony-species
+
+// Overwriting an array instance's __proto__ updates the protector
+
+let x = [];
+
+assertEquals(Array, x.map(()=>{}).constructor);
+assertEquals(Array, x.filter(()=>{}).constructor);
+assertEquals(Array, x.slice().constructor);
+assertEquals(Array, x.splice().constructor);
+assertEquals(Array, x.concat([1]).constructor);
+assertEquals(1, x.concat([1])[0]);
+
+class MyArray extends Array { }
+
+x.__proto__ = MyArray.prototype;
+
+assertEquals(MyArray, x.map(()=>{}).constructor);
+assertEquals(MyArray, x.filter(()=>{}).constructor);
+assertEquals(MyArray, x.slice().constructor);
+assertEquals(MyArray, x.splice().constructor);
+assertEquals(MyArray, x.concat([1]).constructor);
+assertEquals(1, x.concat([1])[0]);