fix csslib and watcher warnings

R=rnystrom@google.com, terry@google.com

Review URL: https://codereview.chromium.org//50703011

git-svn-id: https://dart.googlecode.com/svn/branches/bleeding_edge/dart/pkg/watcher@29632 260f80e4-7a28-3924-810f-c04153c831b5

pkgs/watcher/lib/src/directory_watcher.dart

diff --git a/pkgs/watcher/lib/src/directory_watcher.dart b/pkgs/watcher/lib/src/directory_watcher.dart
index 3b39f5a..679e227 100644
--- a/pkgs/watcher/lib/src/directory_watcher.dart
+++ b/pkgs/watcher/lib/src/directory_watcher.dart
@@ -156,7 +156,7 @@
     if (file == null) return _completePoll();
 
     return getModificationTime(file).then((modified) {
-      if (_checkForCancel()) return;
+      if (_checkForCancel()) return null;
 
       var lastStatus = _statuses[file];
 
@@ -164,7 +164,7 @@
       if (lastStatus != null && lastStatus.modified == modified) {
         // The file is still here.
         _polledFiles.add(file);
-        return;
+        return null;
       }
 
       return _hashFile(file).then((hash) {