Make "--no-packages-dir" the default.

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 c9d2f7dacdb17f3d624e5df001d86baa895a48f4..a9eef560f3320b841f7868a3ac431a8630812a12 100644
--- a/lib/src/command/downgrade.dart
+++ b/lib/src/command/downgrade.dart
@@ -26,7 +26,7 @@ class DowngradeCommand extends PubCommand {
         help: "Report what dependencies would change but don't change any.");
 
     argParser.addFlag('packages-dir',
-        negatable: true, defaultsTo: true,
+        negatable: true,
         help: "Generate a packages/ directory when installing packages.");
   }