issue 17992 Added extra params to message

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 737c72224f88ee21172239a971e776371ddf7dba..298269a6c5a5232518c48727c2eb9221d69e97c8 100644
--- a/pkg/shelf/lib/src/message.dart
+++ b/pkg/shelf/lib/src/message.dart
@@ -23,8 +23,17 @@ abstract class 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 parameters for handlers to pass data to downstream handlers

[nweiz, 2014/04/08 21:33:50]

I don't know that everyone will have the same intuition for which direction is
"upstream" and which is "downstream". It's especially confusing because during
request handling, this will be passed to inner handlers, but during response
handling it will be passed to outer handlers. Maybe just "other handlers" is
best.

[andersmholmgren, 2014/04/08 22:16:36]

My bad, clearly I was thinking of request context when I wrote this comment.
Will amend

+  /// TODO: less sucky name
+  ///
+  /// The value is immutable.
+  final Map<String, Object> extraParams;

[nweiz, 2014/04/08 21:33:50]

I like "context" as a name here.

[andersmholmgren, 2014/04/08 22:16:36]

+1. That's the best name. It came to me a day after submitting this but I
couldn't figure out how to add a comment to my own review.

Other options are handlerContext, appContext etc

+
+
+  Message(UnmodifiableMapView<String, String> headers, this._body,
+          [ UnmodifiableMapView<String, Object> extraParams ])
+      : this.headers = headers,
+        this.extraParams = extraParams;
 
   /// The contents of the content-length field in [headers].
   ///