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

lib/src/command/downgrade.dart

Index: lib/src/command/downgrade.dart
diff --git a/lib/src/command/downgrade.dart b/lib/src/command/downgrade.dart
index fd70963158ab54a7426c038ac798d540bf9d662c..c9d2f7dacdb17f3d624e5df001d86baa895a48f4 100644
--- a/lib/src/command/downgrade.dart
+++ b/lib/src/command/downgrade.dart
@@ -24,12 +24,19 @@ class DowngradeCommand extends PubCommand {
 
     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.");