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

pkg/shelf/test/hijack_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.hijack_test;
+
+import 'dart:async';
+
+import 'package:unittest/unittest.dart';
+import 'package:shelf/shelf.dart';
+
+import 'test_util.dart';
+
+void main() {
+  test('hijacking a non-hijackable request throws a StateError', () {
+    expect(() => new Request('GET', LOCALHOST_URI).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);
+    }));
+
+    expect(() => request.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 a hijackable request twice throws a StateError', () {
+    // Assert that the [onHijack] callback is only called once.
+    var request = new Request('GET', LOCALHOST_URI,
+        onHijack: expectAsync((_) => null, count: 1));
+
+    expect(() => request.hijack((_, __) => null),
+        throwsA(new isInstanceOf<HijackException>()));
+
+    expect(() => request.hijack((_, __) => null), throwsStateError);
+  });
+}