Add a context field to Message in shelf.

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;
 
+  /// Extra context that can be used by for middleware and handlers.
+  ///
+  /// For requests, this is used to pass data to inner middleware and handlers;
+  /// for responses, it's used to pass data to outer middleware and handlers.
+  ///
+  /// Context properties that are used by a particular package should begin with
+  /// that package's name followed by a period. For example, if [logRequests]
+  /// wanted to take a prefix, its property name would be `"shelf.prefix"`,
+  /// since it's in the `shelf` package.
+  ///
+  /// The value is immutable.
+  final Map<String, Object> context;
+
   /// The streaming body of the message.
   ///
   /// This can be read via [read] or [readAsString].
   final Stream<List<int>> _body;
 
   /// Creates a new [Message].
   ///
   /// If [headers] is `null`, it is treated as empty.
-  Message(this._body, {Map<String, String> headers})
-      : this.headers = _getIgnoreCaseMapView(headers);
+  Message(this._body, {Map<String, String> headers,
+        Map<String, Object> context})
+      : this.headers = _getIgnoreCaseMapView(headers),
+        this.context = new pc.UnmodifiableMapView(
+            context == null ? const {} : new Map.from(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);
 }