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

pkg/shelf/test/create_middleware_test.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.create_middleware_test;
 
 import 'dart:async';
 
 import 'package:shelf/shelf.dart';
 import 'package:unittest/unittest.dart';
@@ skipping 184 matching lines @@
         throw error;
       });
 
       var handler = const Pipeline().addMiddleware(middleware)
           .addHandler((request) {
         throw 'bad handler';
       });
 
       expect(makeSimpleRequest(handler), throwsA('bad handler'));
     });
+
+    test("doesn't handle HijackException", () {
+      var middleware = createMiddleware(errorHandler: (error, stack) {
+        expect(false, "error handler shouldn't be called");
+      });
+
+      var handler = const Pipeline().addMiddleware(middleware)
+          .addHandler((request) => throw const HijackException());
+
+      expect(makeSimpleRequest(handler),
+          throwsA(new isInstanceOf<HijackException>()));
+    });
   });
 }
 
 _failHandler(Request request) => fail('should never get here');
 
 final Response _middlewareResponse =
     new Response.ok('middleware content', headers: {'from': 'middleware'});