Fix some analyzer warnings and hints in pkg/watcher.

pkg/watcher/lib/src/directory_watcher/mac_os.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.mac_os;
 
 import 'dart:async';
 import 'dart:io';
 
 import '../constructable_file_system_event.dart';
 import '../path_set.dart';
 import '../utils.dart';
 import '../watch_event.dart';
 import 'resubscribable.dart';
 
-import 'package:path/path.dart' as p;
-
 /// Uses the FSEvents subsystem to watch for filesystem events.
 ///
 /// FSEvents has two main idiosyncrasies that this class works around. First, it
 /// will occasionally report events that occurred before the filesystem watch
 /// was initiated. Second, if multiple events happen to the same file in close
 /// succession, it won't report them in the order they occurred. See issue
 /// 14373.
 ///
 /// This also works around issues 14793, 14806, and 14849 in the implementation
 /// of [Directory.watch].
@@ skipping 302 matching lines @@
     // Don't emit ADD events for files that we already know about. Such an event
     // probably comes from FSEvents reporting an add that happened prior to the
     // watch beginning.
     if (type == ChangeType.ADD && _files.contains(path)) return;
 
     _eventsController.add(new WatchEvent(type, path));
   }
 
   /// Emit an error, then close the watcher.
   void _emitError(error, StackTrace stackTrace) {
-    _eventsController.add(error, stackTrace);
+    _eventsController.addError(error, stackTrace);
     close();
   }
 
   /// Like [Stream.listen], but automatically adds the subscription to
   /// [_subscriptions] so that it can be canceled when [close] is called.
   void _listen(Stream stream, void onData(event), {Function onError,
       void onDone(), bool cancelOnError}) {
     var subscription;
     subscription = stream.listen(onData, onError: onError, onDone: () {
       _subscriptions.remove(subscription);
       if (onDone != null) onDone();
     }, cancelOnError: cancelOnError);
     _subscriptions.add(subscription);
   }
 }