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

utils/pub/lock_file.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('lock_file');
 
+#import('dart:json');
 #import('package.dart');
 #import('source_registry.dart');
 #import('utils.dart');
 #import('version.dart');
 #import('yaml/yaml.dart');
 
 /**
  * A parsed and validated `pubspec.lock` file.
  */
 class LockFile {
@@ skipping 36 matching lines @@
           throw new FormatException(
               'Could not find a source named $sourceName.');
         }
         var source = sources[sourceName];
 
         // Parse the description.
         if (!spec.containsKey('description')) {
           throw new FormatException('Package $name is missing a description.');
         }
         var description = spec['description'];
-        source.validateDescription(description);
+        source.validateDescription(description, fromLockFile: true);
 
         var id = new PackageId(source, version, description);
 
         // Validate the name.
         if (name != id.name) {
           throw new FormatException(
             "Package name $name doesn't match ${id.name}.");
         }
 
         packages[name] = id;
       });
     }
 
     return new LockFile._(packages);
   }
+
+  /**
+   * Returns the serialized YAML text of the lock file.
+   */
+  String serialize() {
+    var packagesObj = <Map>{};
+    packages.forEach((name, id) {
+      packagesObj[name] = {
+        'version': id.version.toString(),
+        'source': id.source.name,
+        'description': id.description
+      };
+    });
+
+    // TODO(nweiz): Serialize using the YAML library once it supports
+    // serialization. For now, we use JSON, since it's a subset of YAML anyway.
+    return JSON.stringify({'packages': packagesObj});
+  }
 }