fix csslib and watcher warnings

pkg/watcher/lib/src/directory_watcher.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;
 
 import 'dart:async';
 import 'dart:io';
 
 import 'package:crypto/crypto.dart';
@@ skipping 138 matching lines @@
 
   /// Processes [file] to determine if it has been modified since the last
   /// time it was scanned.
   Future _processFile(String file) {
     assert(_state != _WatchState.UNSUBSCRIBED);
 
     // `null` is the sentinel which means the directory listing is complete.
     if (file == null) return _completePoll();
 
     return getModificationTime(file).then((modified) {
-      if (_checkForCancel()) return;
+      if (_checkForCancel()) return null;
 
       var lastStatus = _statuses[file];
 
       // If its modification time hasn't changed, assume the file is unchanged.
       if (lastStatus != null && lastStatus.modified == modified) {
         // The file is still here.
         _polledFiles.add(file);
-        return;
+        return null;
       }
 
       return _hashFile(file).then((hash) {
         if (_checkForCancel()) return;
 
         var status = new _FileStatus(modified, hash);
         _statuses[file] = status;
         _polledFiles.add(file);
 
         // Only notify while in the watching state.
@@ skipping 94 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);
 }