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..f3ba22fe1bdeb126802e7050cd9327c3fb679a15 100644
--- a/pkg/shelf/lib/src/message.dart
+++ b/pkg/shelf/lib/src/message.dart
@@ -23,8 +23,18 @@ 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 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;
+
+
+  Message(UnmodifiableMapView<String, String> headers, this._body,
+          [ UnmodifiableMapView<String, Object> context ])
+      : this.headers = headers,
+        this.context = context;
 
   /// The contents of the content-length field in [headers].
   ///