Fix some analyzer warnings and hints in pkg/watcher.

pkg/watcher/lib/src/directory_watcher/polling.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.
 
 library watcher.directory_watcher.polling;
 
 import 'dart:async';
 import 'dart:io';
 
 import 'package:crypto/crypto.dart';
 
 import '../async_queue.dart';
-import '../directory_watcher.dart';
 import '../stat.dart';
 import '../utils.dart';
 import '../watch_event.dart';
 import 'resubscribable.dart';
 
 /// Periodically polls a directory for changes.
 class PollingDirectoryWatcher extends ResubscribableDirectoryWatcher {
   /// Creates a new polling watcher monitoring [directory].
   ///
   /// If [_pollingDelay] is passed, it specifies the amount of time the watcher
@@ skipping 184 matching lines @@
 class _FileStatus {
   /// The last time the file was modified.
   DateTime modified;
 
   /// The SHA-1 hash of the contents of the file.
   List<int> hash;
 
   _FileStatus(this.modified, this.hash);
 }