Add typed wrapper functions to delegate classes.

lib/src/typed/future.dart

+// Copyright (c) 2016, 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';
+
+class TypeSafeFuture<T> implements Future<T> {
+  final Future _future;
+
+  TypeSafeFuture(this._future);
+
+  Stream<T> asStream() => _future.then((value) => value as T).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((value) => onValue(value as T), onError: onError);
+
+  Future<T> whenComplete(action()) =>
+      new TypeSafeFuture<T>(_future.whenComplete(action));
+
+  Future<T> timeout(Duration timeLimit, {onTimeout()}) =>
+      new TypeSafeFuture<T>(_future.timeout(timeLimit, onTimeout: onTimeout));
+}