Fix an analyzer error in the linux directory watcher.

R=rnystrom@google.com
BUG=

Review URL: https://codereview.chromium.org//65533005

git-svn-id: https://dart.googlecode.com/svn/branches/bleeding_edge/dart/pkg/watcher@30093 260f80e4-7a28-3924-810f-c04153c831b5

pkgs/watcher/lib/src/directory_watcher/linux.dart

diff --git a/pkgs/watcher/lib/src/directory_watcher/linux.dart b/pkgs/watcher/lib/src/directory_watcher/linux.dart
index 9acecf1..7ac6e6e 100644
--- a/pkgs/watcher/lib/src/directory_watcher/linux.dart
+++ b/pkgs/watcher/lib/src/directory_watcher/linux.dart
@@ -82,7 +82,7 @@
   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();
     });
   }