Cast Stream of Streams (dart-lang/stream_transform#52)

pkgs/stream_transform/test/switch_test.dart

diff --git a/pkgs/stream_transform/test/switch_test.dart b/pkgs/stream_transform/test/switch_test.dart
index dda658c..f5ac19a 100644
--- a/pkgs/stream_transform/test/switch_test.dart
+++ b/pkgs/stream_transform/test/switch_test.dart
@@ -43,6 +43,7 @@
           errors = [];
           isDone = false;
           subscription = outer.stream
+              .cast<Stream>()
               .transform(switchLatest())
               .listen(emittedValues.add, onError: errors.add, onDone: () {
             isDone = true;