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 that asserts the types of the value emitted by [future].

[Lasse Reichstein Nielsen, 2016/04/08 09:20:29]

"asserts" isn't clear.
Maybe: 
  Creates a `Future` wrapper which throws if a value result isn't an instance of
`T`.
(or similar and shorter, so it fits in a line).

[nweiz, 2016/04/11 20:26:16]

Done.

+  ///
+  /// This soundly converts a [Future] without a generic type to a `Future<T>`

[Lasse Reichstein Nielsen, 2016/04/08 09:20:29]

You can't have a Future without a generic type - the Future type is generic, so
all Future types have a type parameter, possibly "dynamic".

The function works for any Future<Type>, not just those that are dynamic. It can
be used to *cast* any Future type to another Future type. (We should have a cast
operator on Future doing this instead:
  Future<S> cast<S>();
and probably have that on all generic collections and boxings).

[nweiz, 2016/04/11 20:26:16]

Done.

+  /// by asserting that its value is an instance of `T` whenever it's accessed.

[Lasse Reichstein Nielsen, 2016/04/08 09:20:29]

You don't "access" a future value, it's push, not pull.
Maybe use "whenever it's provided"?

[nweiz, 2016/04/11 20:26:16]

Done.

+  /// If it's not, it throws a [CastError].

[Lasse Reichstein Nielsen, 2016/04/08 09:20:29]

Two "it"s referring to different things in the same sentence, the last one needs
to be written out.

[nweiz, 2016/04/11 20:26:16]

Done.

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