Change dart:io to use Future for one-shot operations.

utils/tests/pub/pub_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.
 
 #library('pub_tests');
 
 #import('dart:io');
 
 #import('test_pub.dart');
 #import('../../../lib/unittest/unittest.dart');
@@ skipping 119 matching lines @@
     ],
     output: '''
     Dependencies installed!
     ''');
 }
 
 versionCommand() {
   testPub('displays the current version',
     args: ['version'],
     output: VERSION_STRING);
-}
+}