Update Stream.listen doc and use ?? in some places.

sdk/lib/async/future.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;
 
 /**
  * An object representing a delayed computation.
  *
  * A [Future] is used to represent a potential value, or error,
@@ skipping 207 matching lines @@
    * If calling [computation] throws, the created future will complete with the
    * error.
    *
    * See also [Completer] for a way to create and complete a future at a
    * later time that isn't necessarily after a known fixed duration.
    */
   factory Future.delayed(Duration duration, [computation()]) {
     _Future result = new _Future<T>();
     new Timer(duration, () {
       try {
-        result._complete(computation == null ? null : computation());
+        result._complete(computation?.call());
       } catch (e, s) {
         _completeWithErrorCallback(result, e, s);
       }
     });
     return result;
   }
 
   /**
    * Wait for all the given futures to complete and collect their values.
    *
@@ skipping 506 matching lines @@
   if (replacement != null) {
     error = _nonNullError(replacement.error);
     stackTrace = replacement.stackTrace;
   }
   result._completeError(error, stackTrace);
 }
 
 /** Helper function that converts `null` to a [NullThrownError]. */
 Object _nonNullError(Object error) =>
   (error != null) ? error : new NullThrownError();