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

utils/pub/pubspec.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('pubspec');
 
 #import('package.dart');
 #import('source.dart');
 #import('source_registry.dart');
 #import('utils.dart');
@@ skipping 77 matching lines @@
                 'Source name $sourceName must be a string.');
           }
 
           source = sources[sourceName];
           description = spec[sourceName];
         } else {
           throw new FormatException(
               'Dependency specification $spec must be a string or a mapping.');
         }
 
-        source.validateDescription(description);
+        source.validateDescription(description, fromLockFile: false);
 
         dependencies.add(new PackageRef(
             source, versionConstraint, description));
       });
     }
 
     return new Pubspec(version, dependencies);
   }
 }