Add second argument to Future error handlers.

pkg/scheduled_test/lib/src/value_future.dart

Index: pkg/scheduled_test/lib/src/value_future.dart
diff --git a/pkg/scheduled_test/lib/src/value_future.dart b/pkg/scheduled_test/lib/src/value_future.dart
index 8aa2c6287e8e6eb1c1c74d24304b66fa49af8878..a7d65171e45070de6ff01133b1b11430a7877b05 100644
--- a/pkg/scheduled_test/lib/src/value_future.dart
+++ b/pkg/scheduled_test/lib/src/value_future.dart
@@ -30,9 +30,9 @@ class ValueFuture<T> implements Future<T> {
   }
 
   Stream<T> asStream() => _future.asStream();
-  Future catchError(onError(Object error), {bool test(error)}) =>
+  Future catchError(Function onError, {bool test(error)}) =>
     _future.catchError(onError, test: test);
-  Future then(onValue(T value), {onError(Object error)}) =>
+  Future then(onValue(T value), {Function onError}) =>
     _future.then(onValue, onError: onError);
   Future<T> whenComplete(action()) => _future.whenComplete(action);
 }