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

test/get/package_name_test.dart

 // Copyright (c) 2014, 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 'package:scheduled_test/scheduled_test.dart';
 
 import 'package:pub/src/exit_codes.dart' as exit_codes;
 
 import '../descriptor.dart' as d;
 import '../test_pub.dart';
@@ skipping 47 matching lines @@
     d.dir(appPath, [
       d.pubspec({
         "name": "foo.bar.baz",
         "version": "1.0.0"
       }),
       d.libDir("foo.bar.baz", "foo.bar.baz 1.0.0")
     ]).create();
 
     pubGet();
 
-    d.packagesDir({"foo.bar.baz": "1.0.0"}).validate();
+    d.dir(appPath, [
+      d.packagesFile({
+        "foo.bar.baz": "."
+      })
+    ]).validate();
   });
 }