Fix missing_return violation newly enforced in Dart ~2.3.2-dev.0.1 (dart-lang/test_descriptor#20)

pkgs/test_descriptor/lib/src/directory_descriptor.dart

diff --git a/pkgs/test_descriptor/lib/src/directory_descriptor.dart b/pkgs/test_descriptor/lib/src/directory_descriptor.dart
index 4674a10..d6ba56a 100644
--- a/pkgs/test_descriptor/lib/src/directory_descriptor.dart
+++ b/pkgs/test_descriptor/lib/src/directory_descriptor.dart
@@ -52,6 +52,7 @@
                 p.basename(entity.path), entity.readAsBytesSync());
           }
           // Ignore broken symlinks.
+          return null;
         }).where((path) => path != null));
   }