Add methods to Result. Bump version to 2.0 and remove deprecated features.

lib/src/result/release_sink.dart

 // Copyright (c) 2016, 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.
 
 import 'dart:async';
 
-import '../result.dart';
+import 'result.dart';
 
-/// Use [Result.captureSinkTransformer].
-@Deprecated("Will be removed in async 2.0.0.")
+/// Used by [Result.releaseSink].
 class ReleaseSink<T> implements EventSink<Result<T>> {
   final EventSink _sink;
 
   ReleaseSink(EventSink<T> sink) : _sink = sink;
 
   void add(Result<T> result) {
-    if (result.isValue) {
-      _sink.add(result.asValue.value);
-    } else {
-      var error = result.asError;
-      _sink.addError(error.error, error.stackTrace);
-    }
+    result.addTo(_sink);
   }
 
   void addError(Object error, [StackTrace stackTrace]) {
     // Errors may be added by intermediate processing, even if it is never
     // added by CaptureSink.
     _sink.addError(error, stackTrace);
   }
 
   void close() {
     _sink.close();
   }
 }