Fix a lockfile freshness checking bug.

lib/src/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.
 
 import 'dart:io';
 
 import 'package:barback/barback.dart';
 import 'package:path/path.dart' as p;
 import 'package:pub_semver/pub_semver.dart';
 
@@ skipping 464 matching lines @@
   PackageDep.magic(String name)
       : super.magic(name),
         constraint = Version.none;
 
   String toString() {
     if (isRoot) return "$name $constraint (root)";
     if (isMagic) return name;
     return "$name $constraint from $source ($description)";
   }
 
-  int get hashCode => name.hashCode ^ source.hashCode;
+  int get hashCode => name.hashCode ^ source.hashCode ^ constraint.hashCode;
 
   bool operator ==(other) {
     // TODO(rnystrom): We're assuming here that we don't need to delve into the
     // description.
     return other is PackageDep &&
            other.name == name &&
            other.source == source &&
            other.constraint == constraint;
   }
 }