model: Add DmNarrow.withUser helper

Author: sirpengi

lib/model/narrow.dart

@@ -161,6 +161,13 @@ class DmNarrow extends Narrow implements SendableNarrow {
     );
   }
 
+  factory DmNarrow.withUser(int userId, {required int selfUserId}) {
+    return DmNarrow(
+      allRecipientIds: {userId, selfUserId}.toList()..sort(),
+      selfUserId: selfUserId,
+    );
+  }
+
   /// The user IDs of everyone in the conversation, sorted.
   ///
   /// Each message in the conversation is sent by one of these users

test/model/narrow_test.dart

@@ -69,6 +69,27 @@ void main() {
         selfUserId: eg.selfUser.userId));
     });
 
+    test('ofUser: same user', () {
+      final actual = DmNarrow.withUser(5, selfUserId: 5);
+      check(actual).equals(DmNarrow(
+        allRecipientIds: [5],
+        selfUserId: 5));
+    });
+
+    test('ofUser: user less than selfUserId', () {
+      final actual = DmNarrow.withUser(3, selfUserId: 5);
+      check(actual).equals(DmNarrow(
+          allRecipientIds: [3, 5],
+          selfUserId: 5));
+    });
+
+    test('ofUser: user greater than selfUserId', () {
+      final actual = DmNarrow.withUser(7, selfUserId: 5);
+      check(actual).equals(DmNarrow(
+          allRecipientIds: [5, 7],
+          selfUserId: 5));
+    });
+
     test('otherRecipientIds', () {
       check(DmNarrow(allRecipientIds: [1, 2, 3], selfUserId: 2))
         .otherRecipientIds.deepEquals([1, 3]);