Modernize the package's style.

lib/src/result/capture_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 CaptureSink<T> implements EventSink<T> {
+  final EventSink _sink;
+
+  CaptureSink(EventSink<Result<T>> sink) : _sink = sink;
+
+  void add(T value) {
+    _sink.add(new Result.value(value));
+  }
+
+  void addError(Object error, [StackTrace stackTrace]) {
+    _sink.add(new Result.error(error, stackTrace));
+  }
+
+  void close() {
+    _sink.close();
+  }
+}