[vm] Improve unwinding from Future.doWhile

Annotate result future as an awaiter-link

CoreLibraryReviewExempt: No API changes, just adding a helper pragma.
Change-Id: I6a6a6964615d701b64fb4ecf2082865f99f5a391
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/451520
Reviewed-by: Lasse Nielsen <[email protected]>
Commit-Queue: Slava Egorov <[email protected]>

Author: mraleph

sdk/lib/async/future.dart

@@ -718,6 +718,7 @@ abstract interface class Future<T> {
   /// // Outputs: 'Finished with 3'
   /// ```
   static Future<void> doWhile(FutureOr<bool> action()) {
+    @pragma('vm:awaiter-link')
     _Future<void> doneSignal = _Future<void>();
     late void Function(bool) nextIteration;
     // Bind this callback explicitly so that each iteration isn't bound in the