msglist [nfc]: Simplify a variable into a const

Author: chrisbobbe

lib/widgets/message_list.dart

@@ -656,16 +656,14 @@ class StreamMessageRecipientHeader extends StatelessWidget {
 
     final subscription = store.subscriptions[message.streamId];
     final Color backgroundColor;
-    final Color contrastingColor;
+    const contrastingColor = Colors.black;
     final Color iconColor;
     if (subscription != null) {
       final swatch = subscription.colorSwatch();
       backgroundColor = swatch.barBackground;
-      contrastingColor = Colors.black;
       iconColor = swatch.iconOnBarBackground;
     } else {
       backgroundColor = _kUnsubscribedStreamRecipientHeaderColor;
-      contrastingColor = Colors.black;
       iconColor = Colors.black;
     }
     final textStyle = TextStyle(