Allow Suites to be added to an Engine over time.

lib/src/util/delegating_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 test.util.delegating_sink;
+
+// TODO(nweiz): Move this into package:async.
+/// An implementation of [Sink] that forwards all calls to a wrapped [Sink].
+///
+/// This can also be used on a subclass to make it look like a normal [Sink].
+class DelegatingSink<T> implements Sink<T> {
+  /// The wrapped [Sink].
+  final Sink _inner;
+
+  DelegatingSink(this._inner);
+
+  void add(T data) {
+    _inner.add(data);
+  }
+
+  void close() {
+    _inner.close();
+  }
+}