Initialize _BroadcastSubscription._eventState to permit inference of non-null value

sdk/lib/async/broadcast_stream_controller.dart

Index: sdk/lib/async/broadcast_stream_controller.dart
diff --git a/sdk/lib/async/broadcast_stream_controller.dart b/sdk/lib/async/broadcast_stream_controller.dart
index 6f4ddab11c5156fe17302360b3052d655d63b7a6..a926f1dd5d74916d4a3b26e9c17faa203c465a9b 100644
--- a/sdk/lib/async/broadcast_stream_controller.dart
+++ b/sdk/lib/async/broadcast_stream_controller.dart
@@ -23,7 +23,7 @@ class _BroadcastSubscription<T> extends _ControllerSubscription<T>
   // TODO(lrn): Use the _state field on _ControllerSubscription to
   // also store this state. Requires that the subscription implementation
   // does not assume that it's use of the state integer is the only use.
-  int _eventState;
+  int _eventState = 0;  // Initialized to help dart2js type inference.
 
   _BroadcastSubscriptionLink _next;
   _BroadcastSubscriptionLink _previous;