System.Threading.Tasks

 namespace System.Threading.Tasks {
+    public static class TaskAsyncEnumerableExtensions {
+        public static ConfiguredAsyncDisposable ConfigureAwait(this IAsyncDisposable source, bool continueOnCapturedContext);
+        public static ConfiguredCancelableAsyncEnumerable<T> ConfigureAwait<T>(this IAsyncEnumerable<T> source, bool continueOnCapturedContext);
+        public static ConfiguredCancelableAsyncEnumerable<T> WithCancellation<T>(this IAsyncEnumerable<T> source, CancellationToken cancellationToken);
+    }
 }