Fix analyzer hints and warnings in test/.

test/serve/native_watch_replaced_file_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 file.
 
 library pub_tests;
 
-import 'dart:async';
 import 'dart:io';
 
 import 'package:path/path.dart' as p;
 import 'package:scheduled_test/scheduled_test.dart';
 
 import '../descriptor.dart' as d;
 import '../test_pub.dart';
 import 'utils.dart';
 
 main() {
@@ skipping 30 matching lines @@
 
      // Read the transformed file to ensure the change is actually noticed by
      // pub and not that we just get the new file contents piped through
      // without pub realizing they've changed.
      waitForBuildSuccess();
      requestShouldSucceed("file.out", "after.out");
 
      endPubServe();
    });
 }