Fix an analyzer error in the linux directory watcher.

pkg/watcher/lib/src/directory_watcher/linux.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.linux;
 
 import 'dart:async';
 import 'dart:io';
 
 import '../directory_watcher.dart';
@@ skipping 64 matching lines @@
     }, onDone: () {
       _waitUntilReady().then((_) => _readyCompleter.complete());
     }, cancelOnError: true);
   }
 
   /// Returns a [Future] that completes once all the subdirectory watchers are
   /// fully initialized.
   Future _waitUntilReady() {
     return Future.wait(_subWatchers.values.map((watcher) => watcher.ready))
         .then((_) {
-      if (_subWatchers.values.every((watcher) => watcher.isReady)) return;
+      if (_subWatchers.values.every((watcher) => watcher.isReady)) return null;
       return _waitUntilReady();
     });
   }
 
   void close() {
     for (var subscription in _subscriptions) {
       subscription.cancel();
     }
     for (var watcher in _subWatchers.values) {
       watcher.close();
@@ skipping 193 matching lines @@
   static const DIRECTORY = const _EntryState._("directory");
 
   const _EntryState._(this._name);
 
   /// Returns [DIRECTORY] if [isDir] is true, and [FILE] otherwise.
   factory _EntryState(bool isDir) =>
       isDir ? _EntryState.DIRECTORY : _EntryState.FILE;
 
   String toString() => _name;
 }