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

pkg/shelf/lib/src/hijack_exception.dart

Index: pkg/shelf/lib/src/hijack_exception.dart
diff --git a/pkg/shelf/lib/src/hijack_exception.dart b/pkg/shelf/lib/src/hijack_exception.dart
new file mode 100644
index 0000000000000000000000000000000000000000..13c505444d64470ae7ccfd338e33eeb3825fc5c3
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+++ b/pkg/shelf/lib/src/hijack_exception.dart
@@ -0,0 +1,21 @@
+// 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_exception;
+
+/// An exception used to indicate that a request has been hijacked.
+///
+/// This shouldn't be captured by any code other than the Shelf adapter that
+/// created the hijackable request. Middleware that captures exceptions should
+/// make sure to pass on HijackExceptions.
+///
+/// See also [Request.hijack].
+class HijackException {
+  const HijackException();
+
+  String toString() =>
+      "A shelf request's underlying data stream was hijacked.\n"
+      "This exception is used for control flow and should only be handled by a "
+        "Shelf adapter.";
+}