Addressed comments from review https://codereview.chromium.org/226263007

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:collection';
 import 'dart:convert';
 
 // TODO(kevmoo): use UnmodifiableMapView from SDK once 1.4 ships
 import 'package:collection/wrappers.dart' as pc;
 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;
 
+  /// 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;
+
+
   /// Creates a new [Message].
   ///
   /// If [headers] is `null`, it is treated as empty.
-  Message(this._body, {Map<String, String> headers})
-      : this.headers = _getIgnoreCaseMapView(headers);
+  /// If [context] is `null`, it is treated as empty.
+  Message(this._body, {Map<String, String> headers, 
+    Map<String, Object> context : const {}})
+      : this.headers = _getIgnoreCaseMapView(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 61 matching lines @@
   // TODO(kevmoo) generalize this model with a 'canonical map' to align with
   // similiar implementation in http pkg [BaseRequest].
   var map = new LinkedHashMap<String, String>(
       equals: (key1, key2) => key1.toLowerCase() == key2.toLowerCase(),
       hashCode: (key) => key.toLowerCase().hashCode);
 
   map.addAll(headers);
 
   return new pc.UnmodifiableMapView<String, String>(map);
 }