narrow: Add DmNarrow.withUsers

Author: sirpengi

lib/model/narrow.dart

@@ -168,6 +168,13 @@ class DmNarrow extends Narrow implements SendableNarrow {
     );
   }
 
+  factory DmNarrow.withUsers(List<int> userIds, {required int selfUserId}) {
+    return DmNarrow(
+      allRecipientIds: {...userIds, 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

@@ -90,6 +90,20 @@ void main() {
           selfUserId: 5));
     });
 
+    test('withUsers: without selfUserId', () {
+      final actual = DmNarrow.withUsers([1, 2], selfUserId: 3);
+      check(actual).equals(DmNarrow(
+          allRecipientIds: [1, 2, 3],
+          selfUserId: 3));
+    });
+
+    test('withUsers: with selfUserId', () {
+      final actual = DmNarrow.withUsers([1, 2, 3], selfUserId: 3);
+      check(actual).equals(DmNarrow(
+          allRecipientIds: [1, 2, 3],
+          selfUserId: 3));
+    });
+
     test('otherRecipientIds', () {
       check(DmNarrow(allRecipientIds: [1, 2, 3], selfUserId: 2))
         .otherRecipientIds.deepEquals([1, 3]);