api [nfc]: Support const constructor for MessageDestination.

Signed-off-by: Zixuan James Li <[email protected]>

Author: PIG208

lib/api/route/messages.dart

@@ -219,14 +219,16 @@ Future<SendMessageResult> sendMessage(
 /// Which conversation to send a message to, in [sendMessage].
 ///
 /// This is either a [StreamDestination] or a [DmDestination].
-sealed class MessageDestination {}
+sealed class MessageDestination {
+  const MessageDestination();
+}
 
 /// A conversation in a stream, for specifying to [sendMessage].
 ///
 /// The server accepts a stream name as an alternative to a stream ID,
 /// but this binding currently doesn't.
 class StreamDestination extends MessageDestination {
-  StreamDestination(this.streamId, this.topic);
+  const StreamDestination(this.streamId, this.topic);
 
   final int streamId;
   final String topic;
@@ -237,7 +239,7 @@ 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 {
-  DmDestination({required this.userIds});
+  const DmDestination({required this.userIds});
 
   final List<int> userIds;
 }

test/api/route/messages_test.dart

@@ -325,7 +325,7 @@ void main() {
     test('smoke', () {
       return FakeApiConnection.with_((connection) async {
         await checkSendMessage(connection,
-          destination: StreamDestination(streamId, topic), content: content,
+          destination: const StreamDestination(streamId, topic), content: content,
           queueId: 'abc:123',
           localId: '456',
           readBySender: true,
@@ -344,7 +344,7 @@ void main() {
     test('to stream', () {
       return FakeApiConnection.with_((connection) async {
         await checkSendMessage(connection,
-          destination: StreamDestination(streamId, topic), content: content,
+          destination: const StreamDestination(streamId, topic), content: content,
           readBySender: true,
           expectedBodyFields: {
             'type': 'stream',
@@ -359,7 +359,7 @@ void main() {
     test('to DM conversation', () {
       return FakeApiConnection.with_((connection) async {
         await checkSendMessage(connection,
-          destination: DmDestination(userIds: userIds), content: content,
+          destination: const DmDestination(userIds: userIds), content: content,
           readBySender: true,
           expectedBodyFields: {
             'type': 'direct',
@@ -373,7 +373,7 @@ void main() {
     test('to DM conversation, with legacy type "private"', () {
       return FakeApiConnection.with_(zulipFeatureLevel: 173, (connection) async {
         await checkSendMessage(connection,
-          destination: DmDestination(userIds: userIds), content: content,
+          destination: const DmDestination(userIds: userIds), content: content,
           readBySender: true,
           expectedBodyFields: {
             'type': 'private',
@@ -388,7 +388,7 @@ void main() {
     test('when readBySender is null, sends a User-Agent we know the server will recognize', () {
       return FakeApiConnection.with_((connection) async {
         await checkSendMessage(connection,
-          destination: StreamDestination(streamId, topic), content: content,
+          destination: const StreamDestination(streamId, topic), content: content,
           readBySender: null,
           expectedBodyFields: {
             'type': 'stream',
@@ -403,7 +403,7 @@ void main() {
     test('legacy: when server does not support readBySender, sends a User-Agent the server will recognize', () {
       return FakeApiConnection.with_(zulipFeatureLevel: 235, (connection) async {
         await checkSendMessage(connection,
-          destination: StreamDestination(streamId, topic), content: content,
+          destination: const StreamDestination(streamId, topic), content: content,
           readBySender: true,
           expectedBodyFields: {
             'type': 'stream',