Make stdout and stderr async.

tests/standalone/io/process_sync_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.
 //
 // Utility script to generate some output on stdout and stderr.
 
+import "dart:async";
 import "dart:math";
 import "dart:io";
 
 main() {
   var options = new Options();
   var blockCount = int.parse(options.arguments[0]);
   var stdoutBlockSize = int.parse(options.arguments[1]);
   var stderrBlockSize = int.parse(options.arguments[2]);
   var stdoutBlock =
       new String.fromCharCodes(new List.filled(stdoutBlockSize, 65));
   var stderrBlock =
       new String.fromCharCodes(new List.filled(stderrBlockSize, 66));
   for (int i = 0; i < blockCount; i++) {
     stdout.write(stdoutBlock);
     stderr.write(stderrBlock);
   }
-  exit(int.parse(options.arguments[3]));
+  Future.wait([stdout.close(), stderr.close()]).then((_) {
+    exit(int.parse(options.arguments[3]));
+  });
 }