Add Stream.empty.

sdk/lib/async/future.dart

Index: sdk/lib/async/future.dart
diff --git a/sdk/lib/async/future.dart b/sdk/lib/async/future.dart
index 5d6a65009a4afae64ff5d150d7d8816bc876526f..13de367c5aa6c39af4adf7c64765635211f247c0 100644
--- a/sdk/lib/async/future.dart
+++ b/sdk/lib/async/future.dart
@@ -181,6 +181,7 @@ abstract class Future<T> {
    * 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);
   }