Fix Array.prototype.concat for arguments object with getter.

test/mjsunit/regress/regress-crbug-516775.js

+// Copyright 2015 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: --allow-natives-syntax --harmony-concat-spreadable
+
+function arguments_with_length_getter(f) {
+  arguments.__defineGetter__('length', f);
+  return arguments;
+}
+
+var count = 0;
+function increment_count_return() { count++; return "boom"; }
+function increment_count_throw() { count++; throw "boom"; }
+
+// Do not read the length of an arguments object on the prototype chain of
+// an array.
+var a1 = [];
+%NormalizeElements(a1);
+a1.__proto__ = arguments_with_length_getter(increment_count_return);
+[].concat(a1);
+assertEquals(0, count);
+
+var a2 = [];
+%NormalizeElements(a2);
+a2.__proto__ = arguments_with_length_getter(increment_count_throw);
+[].concat(a2);
+assertEquals(0, count);
+
+// Do read the length of an arguments object if spreadable.
+var a3 = arguments_with_length_getter(increment_count_return);
+a3[Symbol.isConcatSpreadable] = true;
+[].concat(a3);
+assertEquals(1, count);
+
+var a4 = arguments_with_length_getter(increment_count_throw);
+a4[Symbol.isConcatSpreadable] = true;
+assertThrows(function() { [].concat(a4); });
+assertEquals(2, count);
+
+// Do read the length of an arguments object on the prototype chain of
+// an object.
+var a5 = {};
+a5.__proto__ = arguments_with_length_getter(increment_count_return);
+a5[Symbol.isConcatSpreadable] = true;
+[].concat(a5);
+assertEquals(3, count);
+
+var a6 = {};
+a6.__proto__ = arguments_with_length_getter(increment_count_throw);
+a6[Symbol.isConcatSpreadable] = true;
+assertThrows(function() { [].concat(a6); });
+assertEquals(4, count);