Fix most strong mode warnings.

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';
 
 /// A wrapper that forwards calls to a [Future].
 class DelegatingFuture<T> implements Future<T> {
   /// The wrapped [Future].
-  final Future _future;
+  final Future<T> _future;

[Lasse Reichstein Nielsen, 2016/03/29 21:53:59]

I would have thought that this was a feature - you could use the delegating
future to cast a Future<Subclass> to a Future<Superclass> (or vice versa if you
happen to know that the result is still valid).

[nweiz, 2016/03/30 00:57:18]

You're still allowed to pass Future<Subclass> to a function that declares
Future<Superclass>, or vice versa, even in strong mode. What you aren't allowed
to do is pass in a value whose static type is Future<dynamic> or
Future<UnrelatedType>.

If you have special knowledge that your Future<dynamic> will actually emit a T,
that's what asserting wrappers like the ones I added to collection are for. I
intend to add those here as well in a future CL.

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