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

pkg/shelf/test/create_middleware_test.dart

Index: pkg/shelf/test/create_middleware_test.dart
diff --git a/pkg/shelf/test/create_middleware_test.dart b/pkg/shelf/test/create_middleware_test.dart
index 5c35f4d4b10fc0a8d2fe90edd05b8741844efcbb..ee01d07d99ca10a34141f636c82e1dd0c57bd4da 100644
--- a/pkg/shelf/test/create_middleware_test.dart
+++ b/pkg/shelf/test/create_middleware_test.dart
@@ -202,6 +202,18 @@ void main() {
 
       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>()));
+    });
   });
 }