Add missing file (missed it in last round).

tests/isolate/issue_21398_child_isolate1.dart

Index: tests/isolate/issue_21398_child_isolate1.dart
===================================================================
--- tests/isolate/issue_21398_child_isolate1.dart	(revision 0)
+++ tests/isolate/issue_21398_child_isolate1.dart	(working copy)
@@ -0,0 +1,12 @@
+// Copyright (c) 2015, 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:isolate';
+import "package:expect/expect.dart";
+
+main(List<String> args, message) {
+  var sendPort1 = args[0] as SendPort;
+  var sendPort2 = args[1] as SendPort;
+  sendPort2.send(sendPort1);
+}