Fix all strong-mode warnings.

test/transformer/lazy_check_content_and_rename.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.
 
 library barback.test.transformer.lazy_check_content_and_rename;
 
-import 'dart:async';
-
 import 'package:barback/barback.dart';
 
 import 'declaring_check_content_and_rename.dart';
 
 class LazyCheckContentAndRenameTransformer
     extends DeclaringCheckContentAndRenameTransformer
     implements LazyTransformer {
   LazyCheckContentAndRenameTransformer({String oldExtension,
         String oldContent, String newExtension, String newContent})
       : super(oldExtension: oldExtension, oldContent: oldContent,
               newExtension: newExtension, newContent: newContent);
 }