fix strong mode errors for 1.19.0-dev.4.0

lib/src/typed/future.dart

Index: lib/src/typed/future.dart
diff --git a/lib/src/typed/future.dart b/lib/src/typed/future.dart
index 4deb4a9e5de8980ad2b0ab0f750a80bf62be073d..f53ec5f9e086ed967eeb054a3658b39fc169c7b3 100644
--- a/lib/src/typed/future.dart
+++ b/lib/src/typed/future.dart
@@ -14,8 +14,8 @@ class TypeSafeFuture<T> implements Future<T> {
   Future<T> catchError(Function onError, {bool test(Object error)}) async =>
       new TypeSafeFuture<T>(_future.catchError(onError, test: test));
 
-  Future/*<S>*/ then/*<S>*/(onValue(T value), {Function onError}) =>
-      _future.then/*<S>*/((value) => onValue(value as T), onError: onError);
+  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));