Mention CancelableCompleter in the CHANGELOG.

lib/src/cancelable_operation.dart

 // Copyright (c) 2015, 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.
 
 library async.cancelable_operation;
 
 import 'dart:async';
 
 import 'package:async/async.dart';
 
-/// An asynchronuos operation that can be cancelled.
+/// An asynchronous operation that can be cancelled.
 ///
 /// The value of this operation is exposed as [value]. When this operation is
 /// cancelled, [value] won't complete either successfully or with an error. If
 /// [value] has already completed, cancelling the operation does nothing.
 class CancelableOperation<T> {
   /// The completer that produced this operation.
   ///
   /// This is canceled when [cancel] is called.
   final CancelableCompleter<T> _completer;
 
@@ skipping 117 matching lines @@
     _inner.completeError(error, stackTrace);
   }
 
   /// Cancel the completer.
   Future _cancel() => _cancelMemo.runOnce(() {
     if (_inner.isCompleted) return null;
     _isCanceled = true;
     if (_onCancel != null) return _onCancel();
   });
 }