Add regression test for http://dartbug.com/21220.

tests/language/vm/issue21220_vm_test.dart

+// Copyright (c) 2014, the Dart project authors.  Please see the AUTHORS file
+// for details. All rights reserved. Use of this source code is governed by a
+// BSD-style license that can be found in the LICENSE file.
+
+import 'dart:typed_data';
+import 'package:expect/expect.dart';
+
+main() {
+  final val = (0xffb15062).toSigned(32);
+  final arr = new Int32x4List(1);
+  arr[0] = new Int32x4(val, val, val, val);
+  Expect.equals(val, arr[0].x);
+  Expect.equals(val, arr[0].y);
+  Expect.equals(val, arr[0].z);
+  Expect.equals(val, arr[0].w);
+}
+