Fix test to copy scripts into the generated directories.

tests/isolate/issue_21398_parent_isolate1_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_isolate1.dart
+// OtherScripts=issue_21398_child_isolate11.dart
+
 import 'dart:isolate';
 import 'dart:async';
 import "package:expect/expect.dart";
 
 class FromMainIsolate {
   String toString() => 'from main isolate';
   int get fld => 10;
 }
 
 func1Child(args) {
@@ skipping 140 matching lines @@
       );
     }
   );
 }
 
 
 main() {
   spawnFuncTest();
   spawnUriTest();
 }