fix strong mode errors for 1.19.0-dev.4.0

lib/src/delegate/future.dart

Index: lib/src/delegate/future.dart
diff --git a/lib/src/delegate/future.dart b/lib/src/delegate/future.dart
index 08b22e8c9690ca4470cc98df854690cdb6ae9a82..8b81076b247fa585d316bb5273089987e7b18201 100644
--- a/lib/src/delegate/future.dart
+++ b/lib/src/delegate/future.dart
@@ -27,8 +27,8 @@ class DelegatingFuture<T> implements Future<T> {
   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);