additional SDK fixes. This gets all of them, once we upgrade to new analyzer

tool/input_sdk/lib/async/broadcast_stream_controller.dart

Index: tool/input_sdk/lib/async/broadcast_stream_controller.dart
diff --git a/tool/input_sdk/lib/async/broadcast_stream_controller.dart b/tool/input_sdk/lib/async/broadcast_stream_controller.dart
index e71d3a3deeaca3b83b2a75cf8ebedee030e08762..9a9df84010030f5be6b94856d280db3257ceec08 100644
--- a/tool/input_sdk/lib/async/broadcast_stream_controller.dart
+++ b/tool/input_sdk/lib/async/broadcast_stream_controller.dart
@@ -201,7 +201,8 @@ abstract class _BroadcastStreamController<T>
     return subscription;
   }
 
-  Future _recordCancel(StreamSubscription<T> subscription) {
+  Future _recordCancel(StreamSubscription<T> sub) {
+    var subscription = sub as _BroadcastSubscription<T>;
     // If already removed by the stream, don't remove it again.
     if (identical(subscription._next, subscription)) return null;
     assert(!identical(subscription._next, subscription));