narrow [nfc]: Move DmNarrow.withUser factory just below plain constructor

As Greg points out:
  zulip#304 (comment)

> It and `ofMessage` and the plain one are the really
> general-purpose constructors; as the number of special-purpose
> constructors grows, it starts getting pushed down where it's
> harder to spot.

Author: chrisbobbe

lib/model/narrow.dart

@@ -146,24 +146,24 @@ class DmNarrow extends Narrow implements SendableNarrow {
       assert(allRecipientIds.contains(selfUserId)),
       _selfUserId = selfUserId;
 
-  factory DmNarrow.ofMessage(DmMessage message, {required int selfUserId}) {
+  factory DmNarrow.withUser(int userId, {required int selfUserId}) {
     return DmNarrow(
-      allRecipientIds: List.unmodifiable(message.allRecipientIds),
+      allRecipientIds: {userId, selfUserId}.toList()..sort(),
       selfUserId: selfUserId,
     );
   }
 
-  /// A [DmNarrow] from an item in [InitialSnapshot.recentPrivateConversations].
-  factory DmNarrow.ofRecentDmConversation(RecentDmConversation conversation, {required int selfUserId}) {
+  factory DmNarrow.ofMessage(DmMessage message, {required int selfUserId}) {
     return DmNarrow(
-      allRecipientIds: [...conversation.userIds, selfUserId]..sort(),
+      allRecipientIds: List.unmodifiable(message.allRecipientIds),
       selfUserId: selfUserId,
     );
   }
 
-  factory DmNarrow.withUser(int userId, {required int selfUserId}) {
+  /// A [DmNarrow] from an item in [InitialSnapshot.recentPrivateConversations].
+  factory DmNarrow.ofRecentDmConversation(RecentDmConversation conversation, {required int selfUserId}) {
     return DmNarrow(
-      allRecipientIds: {userId, selfUserId}.toList()..sort(),
+      allRecipientIds: [...conversation.userIds, selfUserId]..sort(),
       selfUserId: selfUserId,
     );
   }