Add a bunch of delegates for dart:async types.

lib/src/delegate/event_sink.dart

Index: lib/src/delegate/event_sink.dart
diff --git a/lib/src/delegate/event_sink.dart b/lib/src/delegate/event_sink.dart
new file mode 100644
index 0000000000000000000000000000000000000000..1fe86c15ad23b68ef2246743c41b315b70d16dfc
--- /dev/null
+++ b/lib/src/delegate/event_sink.dart
@@ -0,0 +1,25 @@
+// 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.delegate.event_sink;
+
+import 'dart:async';
+
+/// Simple delegating wrapper around an [EventSink].
+///
+/// Subclasses can override individual methods, or use this to expose only the
+/// [EventSink] methods of a subclass.
+class DelegatingEventSink<T> implements EventSink<T> {
+  final EventSink _sink;
+
+  /// Create a delegating sink forwarding calls to [sink].
+  DelegatingEventSink(EventSink sink) : _sink = sink;
+
+  void add(T data) => _sink.add(data);
+
+  void addError(error, [StackTrace stackTrace]) =>
+      _sink.addError(error, stackTrace);
+
+  void close() => _sink.close();
+}