Make errors from StreamController onCancel calls end up in the returned future.

sdk/lib/async/stream.dart

 // Copyright (c) 2013, 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;
 
 // -------------------------------------------------------------------
 // Core Stream types
 // -------------------------------------------------------------------
 
@@ skipping 1156 matching lines @@
 abstract class StreamSubscription<T> {
   /**
    * Cancels this subscription. It will no longer receive events.
    *
    * May return a future which completes when the stream is done cleaning up.
    * This can be used if the stream needs to release some resources
    * that are needed for a following operation,
    * for example a file being read, that should be deleted afterwards.
    * In that case, the file may not be able to be deleted successfully
    * until the returned future has completed.
+   *
+   * The future will be completed with a `null` value.
+   * If the cleanup throws, which it really shouldn't, the returned future
+   * will be completed with that error.
+   *
    * Returns `null` if there is no need to wait.
    */
   Future cancel();
 
   /** Set or override the data event handler of this subscription. */
   void onData(void handleData(T data));
 
   /**
    * Set or override the error event handler of this subscription.
    *
@@ skipping 297 matching lines @@
 class _ControllerEventSinkWrapper<T> implements EventSink<T> {
   EventSink _sink;
   _ControllerEventSinkWrapper(this._sink);
 
   void add(T data) { _sink.add(data); }
   void addError(error, [StackTrace stackTrace]) {
     _sink.addError(error, stackTrace);
   }
   void close() { _sink.close(); }
 }