Add a bunch of delegates for dart:async types.

lib/src/delegate/future.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.future;
+
+import 'dart:async';
+
+/// A wrapper that forwards calls to a [Future].
+class DelegatingFuture<T> implements Future<T> {
+  /// The wrapped [Future].
+  final Future<T> _future;

[Lasse Reichstein Nielsen, 2015/07/02 09:38:39]

Consider making this a `Future` (without the <T>).
One use of wrapping can be to change the type parameter, for example if someone
gives you a Future<Fruit> but you know it's going to be an apple and you want to
pass it to something requiring a Future<Apple>.

[nweiz, 2015/07/06 20:40:30]

Done.

+
+  DelegatingFuture(this._future);
+
+  Stream<T> asStream() => _future.asStream();
+
+  Future catchError(Function onError, {bool test(error)}) =>
+    _future.catchError(onError, test: test);
+
+  Future then(onValue(T value), {Function onError}) =>
+    _future.then(onValue, onError: onError);
+
+  Future<T> whenComplete(action()) => _future.whenComplete(action);
+
+  Future timeout(Duration timeLimit, {void onTimeout()}) =>
+    _future.timeout(timeLimit, onTimeout: onTimeout);
+}