fix strong mode errors for 1.19.0-dev.4.0

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`
   /// 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<T> catchError(Function onError, {bool test(Object error)}) =>
     _future.catchError(onError, test: test);
 
-  Future/*<S>*/ then/*<S>*/(onValue(T value), {Function onError}) =>
-    _future.then/*<S>*/(onValue, onError: onError);
+  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, {onTimeout()}) =>
     _future.timeout(timeLimit, onTimeout: onTimeout);
 }