Fix newly added regression test that left processes hanging on Windows.

tests/standalone/io/regress_7191_test.dart

 // Copyright (c) 2012, 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.
 
 // Regression test for http://dartbug.com/7191.
 
 // Starts a sub-process which in turn starts another sub-process and then closes
 // its standard output. If handles are incorrectly inherited on Windows, this
 // will lead to a situation where the stdout of the first sub-process is never
 // closed which will make this test hang.
 
 import 'dart:io';
+import 'dart:isolate';
 
 main() {
+  var port = new ReceivePort();
   var options = new Options();
   var executable = options.executable;
   var scriptDir = new Path.fromNative(options.script).directoryPath;
   var script = scriptDir.append('regress_7191_script.dart').toNativePath();
   Process.start(executable, [script]).then((process) {
     process.stdin.write([0]);
     process.stdout.onData = process.stdout.read;
     process.stderr.onData = process.stderr.read;
     process.stdout.onClosed = () {
       process.stdin.write([0]);
     };
+    process.onExit = (exitCode) => port.close();
   });
-}
+}