fix csslib and watcher warnings

pkg/watcher/lib/src/directory_watcher.dart

Index: pkg/watcher/lib/src/directory_watcher.dart
diff --git a/pkg/watcher/lib/src/directory_watcher.dart b/pkg/watcher/lib/src/directory_watcher.dart
index 3b39f5a3eaad4c3afda42caa1006afa9694cd3c9..679e227e8d181cefb7124a272e416824d5b5bc8b 100644
--- a/pkg/watcher/lib/src/directory_watcher.dart
+++ b/pkg/watcher/lib/src/directory_watcher.dart
@@ -156,7 +156,7 @@ class DirectoryWatcher {
     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 @@ class DirectoryWatcher {
       if (lastStatus != null && lastStatus.modified == modified) {
         // The file is still here.
         _polledFiles.add(file);
-        return;
+        return null;
       }
 
       return _hashFile(file).then((hash) {