pkg/shelf: include the original `onHijack` callback for Request.change

pkg/shelf/test/hijack_test.dart

Index: pkg/shelf/test/hijack_test.dart
diff --git a/pkg/shelf/test/hijack_test.dart b/pkg/shelf/test/hijack_test.dart
index fa67a6d63113be887d36a2616f6bbaee02cf775b..ee4255a2bc19f57e6acaf451f815d0885aa251e5 100644
--- a/pkg/shelf/test/hijack_test.dart
+++ b/pkg/shelf/test/hijack_test.dart
@@ -48,4 +48,50 @@ void main() {
 
     expect(() => request.hijack((_, __) => null), throwsStateError);
   });
+
+  group('calling change', () {
+    test('hijacking a non-hijackable request throws a StateError', () {
+      var request = new Request('GET', LOCALHOST_URI);
+      var newRequest = request.change();
+      expect(() => newRequest.hijack((_, __) => null),
+          throwsStateError);
+    });
+
+    test('hijacking a hijackable request throws a HijackException and calls '
+        'onHijack', () {
+      var request = new Request('GET', LOCALHOST_URI,
+          onHijack: expectAsync((callback) {
+        var streamController = new StreamController();
+        streamController.add([1, 2, 3]);
+        streamController.close();
+
+        var sinkController = new StreamController();
+        expect(sinkController.stream.first, completion(equals([4, 5, 6])));
+
+        callback(streamController.stream, sinkController);
+      }));
+
+      var newRequest = request.change();
+
+      expect(() => newRequest.hijack(expectAsync((stream, sink) {
+        expect(stream.first, completion(equals([1, 2, 3])));
+        sink.add([4, 5, 6]);
+        sink.close();
+      })), throwsA(new isInstanceOf<HijackException>()));
+    });
+
+    test('hijacking the original request after calling change throws a '
+        'StateError', () {
+      // Assert that the [onHijack] callback is only called once.
+      var request = new Request('GET', LOCALHOST_URI,
+          onHijack: expectAsync((_) => null, count: 1));
+
+      var newRequest = request.change();
+
+      expect(() => newRequest.hijack((_, __) => null),
+          throwsA(new isInstanceOf<HijackException>()));
+
+      expect(() => request.hijack((_, __) => null), throwsStateError);
+    });
+  });
 }