Implementation of async-await transformation on js ast.

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 1018 matching lines @@
 
   Future cancel() {
     StreamSubscription subscription = _subscription;
     if (_state == _STATE_MOVING) {
       _Future<bool> hasNext = _futureOrPrefetch;
       _clear();
       hasNext._complete(false);
     } else {
       _clear();
     }
-    return subscription.cancel();
+    // If the stream is done, the subscription will have been cleared.
+    return subscription == null
+        ? new _Future<Null>.immediate(null)

[floitsch, 2015/02/02 22:00:12]

This does not belong into this CL.

Create a new CL (with test case), and check with Lasse.
Also just return `null`. (after all, the subscription was already allowed to
return null).

[sigurdm, 2015/02/03 16:59:33]

Ok.

+        : subscription.cancel();
   }
 
   void _onData(T data) {
     if (_state == _STATE_MOVING) {
       _current = data;
       _Future<bool> hasNext = _futureOrPrefetch;
       _futureOrPrefetch = null;
       _state = _STATE_FOUND;
       hasNext._complete(true);
       return;
@@ skipping 23 matching lines @@
       _Future<bool> hasNext = _futureOrPrefetch;
       _clear();
       hasNext._complete(false);
       return;
     }
     _subscription.pause();
     _futureOrPrefetch = null;
     _state = _STATE_EXTRA_DONE;
   }
 }