[nnbd/collection] fix infinite loop in nnbd sdk

See https://dart-ci.appspot.com/log/ddc-nnbd-linux-release-chrome/dartdevk-weak-linux-release-chrome/1460/corelib_2/collection_of_test

Change-Id: I5575cb1db89985f5dc84343412835cccc54b9d47
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/132160
Reviewed-by: Bob Nystrom <rnystrom@google.com>
Commit-Queue: Vijay Menon <vsm@google.com>

sdk_nnbd/lib/collection/queue.dart

@@ -189,7 +189,7 @@ abstract class _DoubleLinkedQueueEntry<E> extends DoubleLinkedQueueEntry<E> {
 
   E _remove();
 
-  E get _element => element;
+  E get _element => super._element as E;
 
   DoubleLinkedQueueEntry<E> nextEntry() {
     _DoubleLinkedQueueEntry<E> entry = _nextLink as _DoubleLinkedQueueEntry<E>;