Fix test to copy scripts into the generated directories.

tests/isolate/issue_21398_parent_isolate_test.dart

 // 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.
 
+// Note: the following comment is used by test.dart to additionally compile the
+// other isolate's code.
+// OtherScripts=issue_21398_child_isolate.dart
+
 import 'dart:isolate';
 import 'dart:async';
 import "package:expect/expect.dart";
 
 class FromMainIsolate {
   String toString() => 'from main isolate';
   int get fld => 10;
 }
 
 funcChild(args) {
@@ skipping 41 matching lines @@
         (msg) {
           Expect.isTrue(msg is FromMainIsolate);
           Expect.equals(10, msg.fld);
           receive2.close();
         },
         onError: (e) => print('$e')
       );
     }
   );
 }