Add Stream.empty.

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 163 matching lines @@
 
   /**
    * A future whose value is available in the next event-loop iteration.
    *
    * If [value] is not a [Future], using this constructor is equivalent
    * to [:new Future<T>.sync(() => value):].
    *
    * Use [Completer] to create a Future and complete it later.
    */
   factory Future.value([value]) {
+    if (value == null) return _nullFuture;

[sra1, 2015/06/15 16:17:40]

What is the purpose of this change?

I think this will make

  new Future<int>.value(null) is Future<String>  -->  true

The change will also break under the stricter type checking of DDC, so will end
up being reverted when the DDC copy of the libraries is merged back.

[Lasse Reichstein Nielsen, 2015/06/15 16:24:42]

Ack, true.
The purpose is entirely to save some space and not create extra objects for
null-futures when they aren't needed. But you are right, it breaks typing. 

We probably don't want to do `if (value == null && T == dynamic)...`.

[Lasse Reichstein Nielsen, 2015/06/16 09:25:12]

Rolling this change back. 

     return new _Future<T>.immediate(value);
   }
 
   /**
    * A future that completes with an error in the next event-loop iteration.
    *
    * If [error] is `null`, it is replaced by a [NullThrownError].
    *
    * Use [Completer] to create a future and complete it later.
    */
@@ skipping 526 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();