pkg/shelf: small fix to make analyzer happy

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 98 matching lines @@
 
 /// Creates a new [Request] from the provided [HttpRequest].
 Request _fromHttpRequest(HttpRequest request) {
   var headers = <String, String>{};
   request.headers.forEach((k, v) {
     // Multiple header values are joined with commas.
     // See http://tools.ietf.org/html/draft-ietf-httpbis-p1-messaging-21#page-22
     headers[k] = v.join(',');
   });
 
-  onHijack(callback) {
-    return request.response
+  void onHijack(callback) {
+    request.response
         .detachSocket(writeHeaders: false)
         .then((socket) => callback(socket, socket));
   }
 
   return new Request(request.method, request.requestedUri,
       protocolVersion: request.protocolVersion,
       headers: headers,
       body: request,
       onHijack: onHijack);
 }
@@ skipping 44 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();
 }