Fix null response handling.

lib/shelf_io.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.
 
 /// A Shelf adapter for handling [HttpRequest] objects from `dart:io`.
 ///
 /// One can provide an instance of [HttpServer] as the `requests` parameter in
 /// [serveRequests].
 ///
 /// This adapter supports request hijacking; see [Request.hijack]. It also
@@ skipping 66 matching lines @@
 
       // If the request wasn't hijacked, we shouldn't be seeing this exception.
       return _logError(
           "Caught HijackException, but the request wasn't hijacked.",
           stackTrace);
     }
 
     return _logError('Error thrown by handler.\n$error', stackTrace);
   }).then((response) {
     if (response == null) {
-      response = _logError('null response from handler.');
+      return _writeResponse(
+          _logError('null response from handler.'), request.response);
     } else if (shelfRequest.canHijack) {
       return _writeResponse(response, request.response);
     }
 
     var message = new StringBuffer()
       ..writeln("Got a response for hijacked request "
           "${shelfRequest.method} ${shelfRequest.requestedUri}:")
       ..writeln(response.statusCode);
     response.headers
         .forEach((key, value) => message.writeln("${key}: ${value}"));
@@ skipping 59 matching lines @@
     chain = new Chain.forTrace(stackTrace);
   }
   chain = chain
       .foldFrames((frame) => frame.isCore || frame.package == 'shelf').terse;
 
   stderr.writeln('ERROR - ${new DateTime.now()}');
   stderr.writeln(message);
   stderr.writeln(chain);
   return new Response.internalServerError();
 }