Start adding support for lock files.

utils/pub/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('source_registry');
 
 #import('source.dart');
 
 /**
  * A class that keeps track of [Source]s used for installing packages.
@@ skipping 32 matching lines @@
   void register(Source source) {
     if (_map.containsKey(source.name)) {
       // TODO(nweiz): Real error-handling system
       throw 'Source registry already has a source named ${source.name}';
     }
 
     _map[source.name] = source;
   }
 
   /**
+   * Returns `true` if there is a source named [name].
+   */
+  bool contains(String name) => _map.containsKey(name);
+
+  /**
    * Returns the source named [name]. Throws an error if no such source has been
    * registered. If [name] is null, returns the default source.
    */
   Source operator[](String name) {
     if (name == null) {
       if (defaultSource != null) return defaultSource;
       // TODO(nweiz): Real error-handling system
       throw 'No default source has been registered';
     }
     if (_map.containsKey(name)) return _map[name];
     throw 'No source named $name is registered';
   }
 }