Remove unnecessary imports (dart-archive/watcher#104)

Author: srawlins

pkgs/watcher/lib/src/directory_watcher.dart

@@ -8,7 +8,6 @@ import '../watcher.dart';
 import 'custom_watcher_factory.dart';
 import 'directory_watcher/linux.dart';
 import 'directory_watcher/mac_os.dart';
-import 'directory_watcher/polling.dart';
 import 'directory_watcher/windows.dart';
 
 /// Watches the contents of a directory and emits [WatchEvent]s when something

pkgs/watcher/lib/src/file_watcher.dart

@@ -7,7 +7,6 @@ import 'dart:io';
 import '../watcher.dart';
 import 'custom_watcher_factory.dart';
 import 'file_watcher/native.dart';
-import 'file_watcher/polling.dart';
 
 /// Watches a file and emits [WatchEvent]s when the file has changed.
 ///

pkgs/watcher/lib/src/resubscribable.dart

@@ -5,7 +5,6 @@
 import 'dart:async';
 
 import '../watcher.dart';
-import 'watch_event.dart';
 
 /// A wrapper for [ManuallyClosedWatcher] that encapsulates support for closing
 /// the watcher when it has no subscribers and re-opening it when it's