Reapply "Add a sleep function to dart:io"

utils/pub/sdk.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.
 
 /// Operations relative to the user's installed Dart SDK.
 library sdk;
 
 import 'dart:io';
 
 import 'package:pathos/path.dart' as path;
@@ skipping 38 matching lines @@
     throw new FormatException("The Dart SDK's 'version' file was not in a "
         "format pub could recognize. Found: $version");
   }
 
   // Semantic versions cannot use "_".
   var build = match[4].replaceAll('_', '.');
 
   return new Version(
       int.parse(match[1]), int.parse(match[2]), int.parse(match[3]),
       build: build);
-}
+}