api [nfc]: Apply constraints from DmMessage.allRecipientIds on DmDestination

This does not change the runtime behavior of this class in production.

Later we will replace DmMessage.allRecipientIds with
DmMessage.destination.userIds.

Author: PIG208

lib/api/route/messages.dart

@@ -1,5 +1,6 @@
 import 'package:json_annotation/json_annotation.dart';
 
+import '../../model/algorithms.dart';
 import '../core.dart';
 import '../exception.dart';
 import '../model/model.dart';
@@ -247,9 +248,17 @@ class StreamDestination extends MessageDestination {
 /// The server accepts a list of Zulip API emails as an alternative to
 /// a list of user IDs, but this binding currently doesn't.
 class DmDestination extends MessageDestination {
-  const DmDestination({required this.userIds});
-
-  final List<int> userIds;
+  DmDestination({required this.userIds})
+    : assert(isSortedWithoutDuplicates(userIds.toList()));
+
+  /// The user IDs of all users in the thread, sorted numerically.
+  ///
+  /// This lists the sender as well as all (other) recipients, and it
+  /// lists each user just once.  In particular the self-user is always
+  /// included.
+  ///
+  /// This is required to have an efficient `length`.
+  final Iterable<int> userIds;
 }
 
 @JsonSerializable(fieldRename: FieldRename.snake)

test/api/route/messages_test.dart

@@ -387,7 +387,7 @@ void main() {
     test('to DM conversation', () {
       return FakeApiConnection.with_((connection) async {
         await checkSendMessage(connection,
-          destination: const DmDestination(userIds: userIds), content: content,
+          destination: DmDestination(userIds: userIds), content: content,
           readBySender: true,
           expectedBodyFields: {
             'type': 'direct',
@@ -401,7 +401,7 @@ void main() {
     test('to DM conversation, with legacy type "private"', () {
       return FakeApiConnection.with_(zulipFeatureLevel: 173, (connection) async {
         await checkSendMessage(connection,
-          destination: const DmDestination(userIds: userIds), content: content,
+          destination: DmDestination(userIds: userIds), content: content,
           readBySender: true,
           expectedBodyFields: {
             'type': 'private',

test/api/route/typing_test.dart

@@ -53,7 +53,7 @@ void main() {
   test('send typing status start for dm', () {
     return FakeApiConnection.with_((connection) {
       return checkSetTypingStatus(connection, TypingOp.start,
-        destination: const DmDestination(userIds: userIds),
+        destination: DmDestination(userIds: userIds),
         expectedBodyFields: {
           'op': 'start',
           'type': 'direct',
@@ -91,7 +91,7 @@ void main() {
   test('legacy: use "private" instead of "direct"', () {
     return FakeApiConnection.with_(zulipFeatureLevel: 173, (connection) {
       return checkSetTypingStatus(connection, TypingOp.start,
-        destination: const DmDestination(userIds: userIds),
+        destination: DmDestination(userIds: userIds),
         expectedBodyFields: {
           'op': 'start',
           'type': 'private',