Add a --[no]-packages-dir flag.

lib/src/command/downgrade.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:async';
 
 import '../command.dart';
 import '../log.dart' as log;
 import '../solver/version_solver.dart';
 
 /// Handles the `downgrade` pub command.
 class DowngradeCommand extends PubCommand {
   String get name => "downgrade";
   String get description =>
       "Downgrade the current package's dependencies to oldest versions.\n\n"
       "This doesn't modify the lockfile, so it can be reset with \"pub get\".";
   String get invocation => "pub downgrade [dependencies...]";
 
   bool get isOffline => argResults['offline'];
 
   DowngradeCommand() {
     argParser.addFlag('offline',
         help: 'Use cached packages instead of accessing the network.');
 
     argParser.addFlag('dry-run', abbr: 'n', negatable: false,
         help: "Report what dependencies would change but don't change any.");
+
+    argParser.addFlag('packages-dir',
+        negatable: true, defaultsTo: true,
+        help: "Generate a packages/ directory when installing packages.");
   }
 
   Future run() async {
     var dryRun = argResults['dry-run'];
     await entrypoint.acquireDependencies(SolveType.DOWNGRADE,
-        useLatest: argResults.rest, dryRun: dryRun);
+        useLatest: argResults.rest,
+        dryRun: dryRun,
+        packagesDir: argResults['packages-dir']);
+
     if (isOffline) {
       log.warning("Warning: Downgrading when offline may not update you to "
           "the oldest versions of your dependencies.");
     }
   }
 }