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

utils/pub/package.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('package');
 
 #import('io.dart');
 #import('pubspec.dart');
 #import('source.dart');
 #import('source_registry.dart');
@@ skipping 132 matching lines @@
     var nameComp = name.compareTo(other.name);
     if (nameComp != 0) return nameComp;
 
     return version.compareTo(other.version);
   }
 
   /**
    * Returns the pubspec for this package.
    */
   Future<Pubspec> describe() => source.describe(this);
+
+  /**
+   * Returns a future that completes to the resovled [PackageId] for this id.
+   */
+  Future<PackageId> get resolved() => source.resolveId(this);
 }
 
 /**
  * A reference to a package. Unlike a [PackageId], a PackageRef may not
  * unambiguously refer to a single package. It may describe a range of allowed
  * packages.
  */
 class PackageRef {
   /**
    * The [Source] used to look up the package.
@@ skipping 20 matching lines @@
 
   String toString() => "$name $constraint from $source ($description)";
 
   /**
    * Returns a [PackageId] generated from this [PackageRef] with the given
    * concrete version.
    */
   PackageId atVersion(Version version) =>
     new PackageId(source, version, description);
 }