Fix an analyzer error in the linux directory watcher.

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

Index: pkg/watcher/lib/src/directory_watcher/linux.dart
diff --git a/pkg/watcher/lib/src/directory_watcher/linux.dart b/pkg/watcher/lib/src/directory_watcher/linux.dart
index 9acecf17bc903fa8606530ab2b4cc27e59cccf74..7ac6e6e087a0e189b6c6607e183b01a362d53b56 100644
--- a/pkg/watcher/lib/src/directory_watcher/linux.dart
+++ b/pkg/watcher/lib/src/directory_watcher/linux.dart
@@ -82,7 +82,7 @@ class _LinuxDirectoryWatcher implements ManuallyClosedDirectoryWatcher {
   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();
     });
   }