[sdk] Updating Stream.contains to accept null values.

Change-Id: Icdb4e3fb04625b8e0ffabe1214b196cd7a9eb1ed
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/138888
Reviewed-by: Lasse R.H. Nielsen <[email protected]>
Commit-Queue: Mark Zhou <[email protected]>

Author: Markzipan

sdk_nnbd/lib/async/stream.dart

@@ -837,7 +837,7 @@ abstract class Stream<T> {
    * the returned future is completed with that error,
    * and processing stops.
    */
-  Future<bool> contains(Object needle) {
+  Future<bool> contains(Object? needle) {
     _Future<bool> future = new _Future<bool>();
     StreamSubscription<T> subscription =
         this.listen(null, onError: future._completeError, onDone: () {