Make pub generate .packages file.

test/get/dry_run_does_not_apply_changes_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 '../descriptor.dart' as d;
 import '../test_pub.dart';
 
 main() {
   initConfig();
   integration("--dry-run shows but does not apply changes", () {
     servePackages((builder) {
       builder.serve("foo", "1.0.0");
     });
 
     d.appDir({
       "foo": "1.0.0"
     }).create();
 
     pubGet(args: ["--dry-run"], output: allOf([
       contains("+ foo 1.0.0"),
       contains("Would change 1 dependency.")
     ]));
 
     d.dir(appPath, [
       // The lockfile should not be created.
       d.nothing("pubspec.lock"),
       // The "packages" directory should not have been generated.
-      d.nothing("packages")
+      d.nothing("packages"),
+      // The ".packages" file should not have been created.
+      d.nothing(".packages"),
     ]).validate();
   });
 }