Correctly handle holes when concat()ing double arrays

test/mjsunit/regress/regress-crbug-403409.js

+// Copyright 2014 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.
+
+Array.prototype[0] = 777;
+var kElements = 10;
+
+var input_array = [];
+for (var i = 1; i < kElements; i++) {
+  input_array[i] = 0.5;
+}
+var output_array = input_array.concat(0.5);
+
+assertEquals(kElements + 1, output_array.length);
+assertEquals(777, output_array[0]);
+for (var j = 1; j < kElements; j++) {
+  assertEquals(0.5, output_array[j]);
+}