[builtins] Fix Array.prototype.concat bug

test/mjsunit/regress/regress-crbug-594574-concat-leak-1.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: --expose-gc
+
+array = new Array(10);
+array[0] = 0.1;
+// array[1] = THE_HOLE, reading through the prototype chain
+array[2] = 2.1;
+array[3] = 3.1;
+
+var copy = array.slice(0, array.length);
+
+// Change the array's prototype.
+var proto = {};
+array.__proto__ = proto;
+
+// Define [1] on the prototype to alter the array during concatenation.
+Object.defineProperty(
+  proto, 1, {
+    get() {
+      // Alter the array.
+      array.length = 1;
+      // Force gc to move the array.
+      gc();
+      return "value from proto";
+    },
+    set(new_value) { }
+});
+
+var concatted_array = Array.prototype.concat.call(array);
+assertEquals(concatted_array[0], 0.1);
+assertEquals(concatted_array[1], "value from proto");
+assertEquals(concatted_array[2], undefined);
+assertEquals(concatted_array[3], undefined);