Fix test to copy scripts into the generated directories.

tests/isolate/issue_21398_parent_isolate2_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=deferred_loaded_lib.dart
+
 import 'dart:isolate';
 import 'dart:async';
 import "package:expect/expect.dart";
 
 import "deferred_loaded_lib.dart" deferred as lib;
 
 // In this test case we send an object created from a deferred library
 // that is loaded in the child isolate but not the parent isolate. The
 // parent isolate does not know about the type of this object and throws
 // an unhandled exception.
@@ skipping 25 matching lines @@
           receivePort.close();
         }
       );
     }
   );
 }
 
 main() {
   helperFunction(); /// 01: runtime error
 }