Make pub strong-mode clean.

lib/src/command/list_package_dirs.dart

 // Copyright (c) 2013, 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:collection/collection.dart';
 import 'package:path/path.dart' as p;
 
 import '../command.dart';
 import '../io.dart';
 import '../log.dart' as log;
 import '../utils.dart';
 
 /// Handles the `list-package-dirs` pub command.
 class ListPackageDirsCommand extends PubCommand {
   String get name => "list-package-dirs";
@@ skipping 36 matching lines @@
     // that's just the pubspec and lockfile.
     output["input_files"] = [
       p.normalize(p.absolute(entrypoint.lockFilePath)),
       p.normalize(p.absolute(entrypoint.pubspecPath))
     ];
 
     log.json.message(output);
   }
 }