fix(Dart1.8): Promise handling

modules/facade/src/async.dart

@@ -12,11 +12,12 @@ class PromiseWrapper {
     return new Future.error(obj);
   }
 
-  static Future all(List<Future> promises){
+  static Future<List> all(List<Future> promises){
     return Future.wait(promises);
   }
 
   static Future then(Future promise, Function success, Function onError){
+    if (success == null) return promise.catchError(onError);
     return promise.then(success, onError: onError);
   }
 }