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';
-
-/// Use [Result.captureSinkTransformer].
-@Deprecated("Will be removed in async 2.0.0.")
-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);
-    }
-  }
-
-  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();
-  }
-}