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

utils/pub/utils.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.
 
 /**
  * Generic utility functions. Stuff that should possibly be in core.
  */
 #library('utils');
 
 #import('dart:crypto');
+#import('dart:isolate');
 
 /** Thrown by methods that parse text when the text isn't a valid. */
 class FormatException implements Exception {
   final String message;
 
   FormatException(this.message);
 
   String toString() => message;
 }
 
@@ skipping 90 matching lines @@
     if (match.end() == str.length) return true;
   }
   return false;
 }
 
 /**
  * Returns the hex-encoded sha1 hash of [source].
  */
 String sha1(String source) =>
   CryptoUtils.bytesToHex(new SHA1().update(source.charCodes()).digest());
+
+/**
+ * Returns a [Future] that completes in [milliSeconds].
+ */
+Future sleep(int milliSeconds) {
+  var completer = new Completer();
+  new Timer(milliSeconds, completer.complete);
+  return completer.future;
+}