Add a context field to Message in shelf.

pkg/shelf/lib/src/request.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.request;
 
 import 'dart:async';
 
 import 'package:http_parser/http_parser.dart';
 
@@ skipping 53 matching lines @@
   /// If [url] and [scriptName] are omitted, they are inferred from
   /// [requestedUri].
   ///
   /// Setting one of [url] or [scriptName] and not the other will throw an
   /// [ArgumentError].
   ///
   /// The default value for [protocolVersion] is '1.1'.
   // TODO(kevmoo) finish documenting the rest of the arguments.
   Request(this.method, Uri requestedUri, {String protocolVersion,
     Map<String, String> headers, Uri url, String scriptName,
-    Stream<List<int>> body})
+    Stream<List<int>> body, Map<String, Object> context})
       : this.requestedUri = requestedUri,
         this.protocolVersion = protocolVersion == null ?
             '1.1' : protocolVersion,
         this.url = _computeUrl(requestedUri, url, scriptName),
         this.scriptName = _computeScriptName(requestedUri, url, scriptName),
         super(body == null ? new Stream.fromIterable([]) : body,
-            headers: headers) {
+            headers: headers, context: context) {
     if (method.isEmpty) throw new ArgumentError('method cannot be empty.');
 
     // TODO(kevmoo) use isAbsolute property on Uri once Issue 18053 is fixed
     if (requestedUri.scheme.isEmpty) {
       throw new ArgumentError('requstedUri must be an absolute URI.');
     }
 
     if (this.scriptName.isNotEmpty && !this.scriptName.startsWith('/')) {
       throw new ArgumentError('scriptName must be empty or start with "/".');
     }
@@ skipping 52 matching lines @@
     return '';
   }
 
   if (url != null && scriptName != null) {
     return scriptName;
   }
 
   throw new ArgumentError(
       'url and scriptName must both be null or both be set.');
 }