Support Request hijacking in Shelf, using a similar API to Rack.

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';
 import 'util.dart';
 
 /// A function which creates a new [Handler] by wrapping a [Handler].
 ///
 /// You can extend the functions of a [Handler] by wrapping it in
 /// [Middleware] that can intercept and process a request before it it sent
 /// to a handler, a response after it is sent by a handler, or both.
 ///
 /// Because [Middleware] consumes a [Handler] and returns a new
 /// [Handler], multiple [Middleware] instances can be composed
 /// together to offer rich functionality.
 ///
 /// Common uses for middleware include caching, logging, and authentication.
 ///
+/// Middleware that captures exceptions should be sure to pass
+/// [HijackException]s on without modification.
+///
 /// A simple [Middleware] can be created using [createMiddleware].
 typedef Handler Middleware(Handler innerHandler);
 
 /// Creates a [Middleware] using the provided functions.
 ///
 /// If provided, [requestHandler] receives a [Request]. It can respond to
 /// the request by returning a [Response] or [Future<Response>].
 /// [requestHandler] can also return `null` for some or all requests in which
 /// case the request is sent to the inner [Handler].
 ///
 /// If provided, [responseHandler] is called with the [Response] generated
 /// by the inner [Handler]. Responses generated by [requestHandler] are not
 /// sent to [responseHandler].
 ///
 /// [responseHandler] should return either a [Response] or
 /// [Future<Response>]. It may return the response parameter it receives or
 /// create a new response object.
 ///
 /// If provided, [errorHandler] receives errors thrown by the inner handler. It
-/// does not receive errors thrown by [requestHandler] or [responseHandler]. It
-/// can either return a new response or throw an error.
+/// 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;
 
   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: errorHandler);
+                onError: (error, stackTrace) {
+          if (error is HijackException) throw error;
+          return errorHandler(error, stackTrace);
+        });
       });
     };
   };
 }