Refactor Source and SourceRegistry.

lib/src/command/cache_list.dart

 // Copyright (c) 2013, 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:convert';
 
 import '../command.dart';
 import '../log.dart' as log;
 import '../source/cached.dart';
 
 /// Handles the `cache list` pub command.
 class CacheListCommand extends PubCommand {
   String get name => "list";
   String get description => "List packages in the system cache.";
   String get invocation => "pub cache list";
   bool get hidden => true;
   bool get takesArguments => false;
 
   void run() {
     // TODO(keertip): Add flag to list packages from non default sources.
     var packagesObj = <String, Map>{};
 
-    var source = cache.sources.defaultSource as CachedSource;
+    var source = cache.defaultSource as CachedSource;
     for (var package in source.getCachedPackages()) {
       var packageInfo = packagesObj.putIfAbsent(package.name, () => {});
       packageInfo[package.version.toString()] = {'location': package.dir};
     }
 
     // TODO(keertip): Add support for non-JSON format and check for --format
     // flag.
     log.message(JSON.encode({'packages': packagesObj}));
   }
 }