fix strong mode errors for 1.19.0-dev.4.0

BUG=
R=jmesserly@google.com

Review URL: https://codereview.chromium.org//2250513003 .
diff --git a/lib/src/delegate/future.dart b/lib/src/delegate/future.dart
index 8b81076..08b22e8 100644
--- a/lib/src/delegate/future.dart
+++ b/lib/src/delegate/future.dart
@@ -27,8 +27,8 @@
   Future<T> 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/*<S>*/ then/*<S>*/(onValue(T value), {Function onError}) =>
+    _future.then/*<S>*/(onValue, onError: onError);
 
   Future<T> whenComplete(action()) => _future.whenComplete(action);
 
diff --git a/lib/src/typed/future.dart b/lib/src/typed/future.dart
index f53ec5f..4deb4a9 100644
--- a/lib/src/typed/future.dart
+++ b/lib/src/typed/future.dart
@@ -14,8 +14,8 @@
   Future<T> catchError(Function onError, {bool test(Object error)}) async =>
       new TypeSafeFuture<T>(_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/*<S>*/ then/*<S>*/(onValue(T value), {Function onError}) =>
+      _future.then/*<S>*/((value) => onValue(value as T), onError: onError);
 
   Future<T> whenComplete(action()) =>
       new TypeSafeFuture<T>(_future.whenComplete(action));