The return-value of 'asFuture' isn't necessarily related to the Stream's type.

sdk/lib/async/stream_impl.dart

Index: sdk/lib/async/stream_impl.dart
diff --git a/sdk/lib/async/stream_impl.dart b/sdk/lib/async/stream_impl.dart
index c23b0f040f6924d75f1b922f150e1d3765b28d3c..6dca1923361e37c961f39500f76ce359038d2504 100644
--- a/sdk/lib/async/stream_impl.dart
+++ b/sdk/lib/async/stream_impl.dart
@@ -203,7 +203,7 @@ class _BufferingStreamSubscription<T> implements StreamSubscription<T>,
   }
 
   Future asFuture([var futureValue]) {
-    _Future<T> result = new _Future<T>();
+    _Future result = new _Future();
 
     // Overwrite the onDone and onError handlers.
     _onDone = () { result._complete(futureValue); };