Add a --pause-after-load flag for debugging.

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();
-  }
-}