[cfe] Update outline expectations

TBR

Change-Id: I44a90a91f8efd749c031c95788a0e7588feb7bec
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/153357
Reviewed-by: Johnni Winther <johnniwinther@google.com>
Reviewed-by: Jens Johansen <jensj@google.com>
Commit-Queue: Johnni Winther <johnniwinther@google.com>

pkg/front_end/testcases/nnbd/issue42540.dart.outline.expect

@@ -5,7 +5,7 @@ import "dart:core" as core;
 
 static method getNull() → dynamic
   ;
-static method fn() → asy::Future<core::Object>
+static method fn() → asy::Future<core::Object> async 
   ;
 static method main() → dynamic
   ;