pkg/shelf: only catch errors in createMiddleware if an errorHandler is provided

pkg/shelf/lib/src/middleware.dart

 // Copyright (c) 2014, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 library shelf.middleware;
 
 import 'request.dart';
 import 'response.dart';
 import 'handler.dart';
 import 'hijack_exception.dart';
@@ skipping 36 matching lines @@
 /// does not receive errors thrown by [requestHandler] or [responseHandler], nor
 /// does it receive [HijackException]s. It can either return a new response or
 /// throw an error.
 Middleware createMiddleware({requestHandler(Request request),
     responseHandler(Response response),
     errorHandler(error, StackTrace stackTrace)}) {
   if (requestHandler == null) requestHandler = (request) => null;
 
   if (responseHandler == null) responseHandler = (response) => response;
 
+  var onError = null;
+  if (errorHandler != null) {
+    onError = (error, stackTrace) {
+      if (error is HijackException) throw error;
+      return errorHandler(error, stackTrace);
+    };
+  }
+
   return (Handler innerHandler) {
     return (request) {
       return syncFuture(() => requestHandler(request)).then((response) {
         if (response != null) return response;
 
         return syncFuture(() => innerHandler(request))
-            .then((response) => responseHandler(response),
-                onError: (error, stackTrace) {
-          if (error is HijackException) throw error;
-          return errorHandler(error, stackTrace);
-        });
+            .then((response) => responseHandler(response), onError: onError);
       });
     };
   };
 }