It was broken. The fix is to ignore the warning.

lib/src/stream_sink_transformer/reject_errors.dart

diff --git a/lib/src/stream_sink_transformer/reject_errors.dart b/lib/src/stream_sink_transformer/reject_errors.dart
index 69c7f96..7e15d26 100644
--- a/lib/src/stream_sink_transformer/reject_errors.dart
+++ b/lib/src/stream_sink_transformer/reject_errors.dart
@@ -113,6 +113,7 @@
     _closed = true;
 
     if (!_canceled) {
+      // ignore: void_checks
       _doneCompleter.complete(_inner.close());
     }
     return done;