Add second argument to Future error handlers.

pkg/barback/lib/src/cancelable_future.dart

Index: pkg/barback/lib/src/cancelable_future.dart
diff --git a/pkg/barback/lib/src/cancelable_future.dart b/pkg/barback/lib/src/cancelable_future.dart
index 043ad6cd8487673ca7caccef7016f92948398b23..284df411362d8dbcfd0af110d37c960a4f7b5625 100644
--- a/pkg/barback/lib/src/cancelable_future.dart
+++ b/pkg/barback/lib/src/cancelable_future.dart
@@ -20,16 +20,16 @@ class CancelableFuture<T> implements Future<T> {
     inner.then((result) {
       if (_canceled) return;
       _completer.complete(result);
-    }).catchError((error) {
+    }).catchError((error, stackTrace) {
       if (_canceled) return;
-      _completer.completeError(error);
+      _completer.completeError(error, stackTrace);
     });
   }
 
   Stream<T> asStream() => _completer.future.asStream();
-  Future catchError(onError(asyncError), {bool test(error)}) =>
+  Future catchError(Function onError, {bool test(error)}) =>
     _completer.future.catchError(onError, test: test);
-  Future then(onValue(T value), {onError(error)}) =>
+  Future then(onValue(T value), {Function onError}) =>
     _completer.future.then(onValue, onError: onError);
   Future<T> whenComplete(action()) => _completer.future.whenComplete(action);