Fix test to copy scripts into the generated directories.

tests/isolate/issue_21398_parent_isolate1_test.dart

Index: tests/isolate/issue_21398_parent_isolate1_test.dart
===================================================================
--- tests/isolate/issue_21398_parent_isolate1_test.dart	(revision 43891)
+++ tests/isolate/issue_21398_parent_isolate1_test.dart	(working copy)
@@ -2,6 +2,11 @@
 // 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.
 
+// Note: the following comment is used by test.dart to additionally compile the
+// other isolate's code.
+// OtherScripts=issue_21398_child_isolate1.dart
+// OtherScripts=issue_21398_child_isolate11.dart
+
 import 'dart:isolate';
 import 'dart:async';
 import "package:expect/expect.dart";