Roll Observatory packages and add a roll script

packages/async/lib/src/delegate/sink.dart

+// 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();
+  }
+}