issue 17992 Added extra params to message

pkg/shelf/lib/src/message.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.message;
 
 import 'dart:async';
 import 'dart:convert';
 
 import 'package:collection/wrappers.dart';
 import 'package:http_parser/http_parser.dart';
 import 'package:stack_trace/stack_trace.dart';
 
 /// Represents logic shared between [Request] and [Response].
 abstract class Message {
   /// The HTTP headers.
   ///
   /// The value is immutable.
   final Map<String, String> headers;
 
   /// The streaming body of the message.
   ///
   /// This can be read via [read] or [readAsString].
   final Stream<List<int>> _body;
 
-  Message(UnmodifiableMapView<String, String> headers, this._body)
-      : this.headers = headers;
+  /// Extra context parameters for handlers to pass data to subsequent handlers.
+  /// Note: for requests this means downstream handlers.
+  /// For responses this means upstream handlers.
+  ///
+  /// The value is immutable.
+  final Map<String, Object> context;
+
+
+  Message(UnmodifiableMapView<String, String> headers, this._body,
+          [ UnmodifiableMapView<String, Object> context ])
+      : this.headers = headers,
+        this.context = context;
 
   /// The contents of the content-length field in [headers].
   ///
   /// If not set, `null`.
   int get contentLength {
     if (_contentLengthCache != null) return _contentLengthCache;
     if (!headers.containsKey('content-length')) return null;
     _contentLengthCache = int.parse(headers['content-length']);
     return _contentLengthCache;
   }
@@ skipping 47 matching lines @@
   /// Otherwise the encoding is taken from the Content-Type header. If that
   /// doesn't exist or doesn't have a "charset" parameter, UTF-8 is used.
   ///
   /// This calls [read] internally, which can only be called once.
   Future<String> readAsString([Encoding encoding]) {
     if (encoding == null) encoding = this.encoding;
     if (encoding == null) encoding = UTF8;
     return Chain.track(encoding.decodeStream(read()));
   }
 }