Improve the detection lockfile freshness.

lib/src/source_registry.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 pub.source_registry;
 
 import 'dart:collection';
 
 import 'package.dart';
 import 'source.dart';
@@ skipping 47 matching lines @@
   /// registered.
   void register(Source source) {
     if (_sources.containsKey(source.name)) {
       throw new StateError('Source registry already has a source named '
           '${source.name}');
     }
 
     _sources[source.name] = source;
   }
 
+  /// Loads the package identified by [id].
+  ///
+  /// Throws an [ArgumentError] if [id] has an invalid source.
+  Package load(PackageId id) {
+    var source = this[id.source];
+    if (source == null) throw new ArgumentError("Unknown source ${id.source}.");
+
+    var dir = source.getDirectory(id);
+    return new Package.load(id.name, dir, this);
+  }
+
   /// Returns the source named [name].
   ///
   /// Returns an [UnknownSource] if no source with that name has been
   /// registered. If [name] is null, returns the default source.
   Source operator[](String name) {
     if (name == null) {
       if (defaultSource != null) return defaultSource;
       throw new StateError('No default source has been registered');
     }
 
     if (_sources.containsKey(name)) return _sources[name];
     return new UnknownSource(name);
   }
 }