Add typed wrapper functions to delegate classes.

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.
 
 import 'dart:async';
 
+import '../typed/future.dart';
+
 /// A wrapper that forwards calls to a [Future].
 class DelegatingFuture<T> implements Future<T> {
   /// The wrapped [Future].
   final Future<T> _future;
 
   DelegatingFuture(this._future);
 
+  /// Creates a wrapper which throws if [future]'s value isn't an instance of
+  /// `T`.
+  ///
+  /// This soundly converts a [Future] to a `Future<T>`, regardless of its
+  /// original generic type, by asserting that its value is an instance of `T`

[floitsch, 2016/04/12 20:03:08]

The text for this one and the EventSink should be similar. From what I can see
they are doing the same thing.

[nweiz, 2016/04/12 20:41:19]

I think it's pretty different. EventSink doesn't ever pass a T to its callers,
so it doesn't make sense to talk about "its value". Instead, it adds additional
assertions about what the user passes in to the sink.

+  /// whenever it's provided. If it's not, the future throws a [CastError].
+  static Future/*<T>*/ typed/*<T>*/(Future future) =>
+      future is Future/*<T>*/ ? future : new TypeSafeFuture/*<T>*/(future);
+
   Stream<T> asStream() => _future.asStream();
 
   Future catchError(Function onError, {bool test(Object error)}) =>
     _future.catchError(onError, test: test);
 
   Future/*<S>*/ then/*<S>*/(/*=S*/ onValue(T value), {Function onError}) =>
     _future.then(onValue, onError: onError);
 
   Future<T> whenComplete(action()) => _future.whenComplete(action);
 
-  Future<T> timeout(Duration timeLimit, {void onTimeout()}) =>

[floitsch, 2016/04/12 20:03:08]

why this change? The value of `onTimeout()` should never be used.

[nweiz, 2016/04/12 20:41:19]

This matches dart:async's declaration. The dart:async docs say:

+  Future<T> timeout(Duration timeLimit, {onTimeout()}) =>
     _future.timeout(timeLimit, onTimeout: onTimeout);
 }