Use a lockfile to persist Pub's installed version constellation.

utils/pub/io.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.
 
 /**
  * Helper functionality to make working with IO easier.
  */
 #library('io');
 
 #import('dart:io');
@@ skipping 111 matching lines @@
         return opened.close().transform((ignore) => file);
     });
   });
 }
 
 /**
  * Asynchronously deletes [file], which can be a [String] or a [File]. Returns a
  * [Future] that completes when the deletion is done.
  */
 Future<Directory> deleteFile(file) {
-  file = _getFile(file);
-  return file.delete();
+  return new File(_getPath(file)).delete();
 }
 
 /**
  * Creates a directory [dir]. Returns a [Future] that completes when the
  * directory is created.
  */
 Future<Directory> createDir(dir) {
   dir = _getDirectory(dir);
   return dir.create();
 }
@@ skipping 327 matching lines @@
   return new Directory(entry);
 }
 
 /**
  * Gets a [Uri] for [uri], which can either already be one, or be a [String].
  */
 Uri _getUri(uri) {
   if (uri is Uri) return uri;
   return new Uri.fromString(uri);
 }