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

test/get/path/shared_dependency_symlink_test.dart

 // Copyright (c) 2013, the Dart project authors.  Please see the AUTHORS d.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 d.file.
 
 import 'package:path/path.dart' as path;
 
 import '../../descriptor.dart' as d;
 import '../../test_pub.dart';
 
 main() {
@@ skipping 20 matching lines @@
     d.dir(appPath, [
       d.appPubspec({
         "foo": {"path": "../foo"},
         "bar": {"path": "../bar"}
       })
     ]).create();
 
     d.dir("link").create();
     scheduleSymlink("shared", path.join("link", "shared"));
 
-    pubGet();
+    pubGet(args: ["--packages-dir"]);
 
     d.dir(packagesPath, [
       d.dir("foo", [d.file("foo.dart", 'main() => "foo";')]),
       d.dir("bar", [d.file("bar.dart", 'main() => "bar";')]),
       d.dir("shared", [d.file("shared.dart", 'main() => "shared";')])
     ]).validate();
   });
 }