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

utils/pub/repo_source.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('dartlang_source');
 
 #import('dart:io');
 #import('dart:json');
 #import('dart:uri');
 #import('io.dart');
@@ skipping 86 matching lines @@
     _parseDescription(description1) == _parseDescription(description2);
 
   /**
    * Ensures that [description] is a valid repo description.
    *
    * There are two valid formats. A plain string refers to a package with the
    * given name from the default repository, while a map with keys "name" and
    * "url" refers to a package with the given name from the repo at the given
    * URL.
    */
-  void validateDescription(description) {
+  void validateDescription(description, [bool fromLockFile=false]) {
     _parseDescription(description);
   }
 
   /**
    * Parses the description blob for a package.
    *
    * If the package parses correctly, this returns a (name, url) pair. If not,
    * this throws a descriptive FormatException.
    */
   Pair<String, String> _parseDescription(description) {
@@ skipping 13 matching lines @@
 
     var name = description["name"];
     if (name is! String) {
       throw new FormatException("The 'name' key must have a string value.");
     }
 
     var url = description.containsKey("url") ? description["url"] : defaultUrl;
     return new Pair<String, String>(name, url);
   }
 }