Fix some analyzer warnings and hints in pkg/watcher.

pkg/watcher/lib/src/directory_watcher/resubscribable.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.resubscribable;
 
 import 'dart:async';
-import 'dart:io';
 
 import '../directory_watcher.dart';
-import '../utils.dart';
 import '../watch_event.dart';
 
 typedef ManuallyClosedDirectoryWatcher WatcherFactory();
 
 /// A wrapper for [ManuallyClosedDirectoryWatcher] that encapsulates support for
 /// closing the watcher when it has no subscribers and re-opening it when it's
 /// re-subscribed.
 ///
 /// It's simpler to implement watchers without worrying about this behavior.
 /// This class wraps a watcher class which can be written with the simplifying
@@ skipping 48 matching lines @@
 /// An interface for watchers with an explicit, manual [close] method.
 ///
 /// See [ResubscribableDirectoryWatcher].
 abstract class ManuallyClosedDirectoryWatcher implements DirectoryWatcher {
   /// Closes the watcher.
   ///
   /// Subclasses should close their [events] stream and release any internal
   /// resources.
   void close();
 }