More minor type improvements in dart:async.

sdk/lib/async/stream_controller.dart

Index: sdk/lib/async/stream_controller.dart
diff --git a/sdk/lib/async/stream_controller.dart b/sdk/lib/async/stream_controller.dart
index 4b88281fd0fe2f605f4553511af567649d53a4ea..1f15703dd2853843a962623a709947828c1301e7 100644
--- a/sdk/lib/async/stream_controller.dart
+++ b/sdk/lib/async/stream_controller.dart
@@ -792,12 +792,10 @@ class _SyncStreamController<T> = _StreamController<T>
 
 typedef _NotificationHandler();
 
-Future _runGuarded(_NotificationHandler notificationHandler) {
-  if (notificationHandler == null) return null;
+void _runGuarded(_NotificationHandler notificationHandler) {
+  if (notificationHandler == null) return;
   try {
-    var result = notificationHandler();
-    if (result is Future) return result;
-    return null;
+    notificationHandler();
   } catch (e, s) {
     Zone.current.handleUncaughtError(e, s);
   }