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

pkg/shelf/test/shelf_io_test.dart

Index: pkg/shelf/test/shelf_io_test.dart
diff --git a/pkg/shelf/test/shelf_io_test.dart b/pkg/shelf/test/shelf_io_test.dart
index bdece66973838198312f835b312828e4d00d1209..520565809d0b95a7b3baef7ea9d990daca3a6744 100644
--- a/pkg/shelf/test/shelf_io_test.dart
+++ b/pkg/shelf/test/shelf_io_test.dart
@@ -185,6 +185,39 @@ void main() {
     });
   });
 
+  test('supports request hijacking', () {
+    _scheduleServer((request) {
+      expect(request.method, 'POST');
+
+      request.hijack(expectAsync((stream, sink) {
+        expect(stream.first, completion(equals("Hello".codeUnits)));
+
+        sink.add((
+            "HTTP/1.1 404 Not Found\r\n"
+            "Date: Mon, 23 May 2005 22:38:34 GMT\r\n"
+            "Content-Length: 13\r\n"
+            "\r\n"
+            "Hello, world!").codeUnits);
+        sink.close();
+      }));
+    });
+
+    return _schedulePost(body: "Hello").then((response) {
+      expect(response.statusCode, HttpStatus.NOT_FOUND);
+      expect(response.headers["date"], "Mon, 23 May 2005 22:38:34 GMT");
+      expect(response.stream.bytesToString(),
+          completion(equals("Hello, world!")));
+    });
+  });
+
+  test('reports an error if a HijackException is thrown without hijacking', () {
+    _scheduleServer((request) => throw const HijackException());
+
+    return _scheduleGet().then((response) {
+      expect(response.statusCode, HttpStatus.INTERNAL_SERVER_ERROR);
+    });
+  });
+
   test('passes asynchronous exceptions to the parent error zone', () {
     return runZoned(() {
       return shelf_io.serve((request) {