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

pkg/shelf/lib/src/message.dart

Index: pkg/shelf/lib/src/message.dart
diff --git a/pkg/shelf/lib/src/message.dart b/pkg/shelf/lib/src/message.dart
index ad64d10ea31604ae930400e60e4cb3955ffcbc5d..693f21a53177fc85a5c82eba281b59c69e68a687 100644
--- a/pkg/shelf/lib/src/message.dart
+++ b/pkg/shelf/lib/src/message.dart
@@ -25,11 +25,22 @@ abstract class 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].
   ///