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

sdk/lib/async/stream_impl.dart

 // Copyright (c) 2012, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 part of dart.async;
 
 /** Abstract and private interface for a place to put events. */
 abstract class _EventSink<T> {
   void _add(T data);
   void _addError(Object error, StackTrace stackTrace);
@@ skipping 185 matching lines @@
     // The user doesn't want to receive any further events. If there is an
     // error or done event pending (waiting for the cancel to be done) discard
     // that event.
     _state &= ~_STATE_WAIT_FOR_CANCEL;
     if (_isCanceled) return _cancelFuture;
     _cancel();
     return _cancelFuture;
   }
 
   Future asFuture([var futureValue]) {
-    _Future<T> result = new _Future<T>();
+    _Future result = new _Future();
 
     // Overwrite the onDone and onError handlers.
     _onDone = () { result._complete(futureValue); };
     _onError = (error, stackTrace) {
       cancel();
       result._completeError(error, stackTrace);
     };
 
     return result;
   }
@@ skipping 869 matching lines @@
 class _EmptyStream<T> extends Stream<T> {
   const _EmptyStream() : super._internal();
   bool get isBroadcast => true;
   StreamSubscription<T> listen(void onData(T data),
                                {Function onError,
                                 void onDone(),
                                 bool cancelOnError}) {
     return new _DoneStreamSubscription<T>(onDone);
   }
 }