Validate exit-code in regress_7191.

tests/standalone/io/regress_7191_script.dart

 // Copyright (c) 2013, 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:io';
 import 'dart:isolate';
 
 main() {
   // Open a port to make the script hang.
   var port = new ReceivePort();
   // Start sub-process when receiving data.
   var subscription;
   subscription = stdin.listen((data) {
     var options = new Options();
     Process.start(options.executable, [options.script]).then((p) {
       p.stdout.listen((_) { });
       p.stderr.listen((_) { });
       // When receiving data again, kill sub-process and exit.
       subscription.onData((data) {
         p.kill();
-        p.exitCode.then(exit);
+        p.exitCode.then((_) => exit(0));
       });
       // Close stdout. If handles are incorrectly inherited this will
       // not actually close stdout and the test will hang.
       stdout.close();
     });
   });
 }