Add a bunch of delegates for dart:async types.

lib/src/delegate/sink.dart

Index: lib/src/delegate/sink.dart
diff --git a/lib/src/delegate/sink.dart b/lib/src/delegate/sink.dart
new file mode 100644
index 0000000000000000000000000000000000000000..5cf4f563adf0956274f4ba7024d763a80431e02b
--- /dev/null
+++ b/lib/src/delegate/sink.dart
@@ -0,0 +1,21 @@
+// 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.sink;
+
+/// Simple delegating wrapper around a [Sink].
+///
+/// Subclasses can override individual methods, or use this to expose only the
+/// [Sink] methods of a subclass.
+class DelegatingSink<T> implements Sink<T> {
+  final Sink _sink;
+
+  /// Create a delegating sink forwarding calls to [sink].
+  DelegatingSink(Sink sink)
+      : _sink = sink;
+
+  void add(T data) => _sink.add(data);
+
+  void close() => _sink.close();
+}