[runtime] Fix Array.prototype.concat with complex @@species

test/mjsunit/regress/regress-crbug-682194.js

+// Copyright 2017 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: --expose-gc
+
+var proxy = new Proxy([], {
+  defineProperty() {
+    w.length = 1;  // shorten the array so the backstore pointer is relocated
+    gc();          // force gc to move the array's elements backstore
+    return Object.defineProperty.apply(this, arguments);
+  }
+});
+
+class MyArray extends Array {
+  // custom constructor which returns a proxy object
+  static get[Symbol.species](){
+    return function() {
+      return proxy;
+    }
+  };
+}
+
+var w = new MyArray(100);
+w[1] = 0.1;
+w[2] = 0.1;
+
+var result = Array.prototype.concat.call(w);
+
+assertEquals(undefined, result[0]);
+assertEquals(0.1, result[1]);
+
+for (var i = 2; i < 200; i++) {
+  assertEquals(undefined, result[i]);
+}