Be sure we don't forward an error to a closed event stream.

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

Index: pkg/watcher/lib/src/directory_watcher/polling.dart
diff --git a/pkg/watcher/lib/src/directory_watcher/polling.dart b/pkg/watcher/lib/src/directory_watcher/polling.dart
index 0e190b02bad943be2b299fd0eae227c80a8b39d1..e50a0c05c148e1601088f94c074e390b9bed0454 100644
--- a/pkg/watcher/lib/src/directory_watcher/polling.dart
+++ b/pkg/watcher/lib/src/directory_watcher/polling.dart
@@ -72,7 +72,9 @@ class _PollingDirectoryWatcher implements ManuallyClosedDirectoryWatcher {
 
   _PollingDirectoryWatcher(this.directory, this._pollingDelay) {
     _filesToProcess = new AsyncQueue<String>(_processFile,
-        onError: _events.addError);
+        onError: (e, stackTrace) {
+      if (!_events.isClosed) _events.addError(e, stackTrace);
+    });
 
     _poll();
   }