api: In sendMessage, use "direct" where possible

Part of #146.

Author: gnprice

lib/api/route/messages.dart

@@ -99,13 +99,14 @@ Future<SendMessageResult> sendMessage(
   String? queueId,
   String? localId,
 }) {
+  final supportsTypeDirect = connection.zulipFeatureLevel! >= 174; // TODO(server-7)
   return connection.post('sendMessage', SendMessageResult.fromJson, 'messages', {
     if (destination is StreamDestination) ...{
       'type': RawParameter('stream'),
       'to': destination.streamId,
       'topic': RawParameter(destination.topic),
     } else if (destination is DmDestination) ...{
-      'type': RawParameter('private'), // TODO(#146): use 'direct' where possible
+      'type': supportsTypeDirect ? RawParameter('direct') : RawParameter('private'),
       'to': destination.userIds,
     } else ...(
       throw Exception('impossible destination') // TODO(dart-3) show this statically

test/api/route/messages_test.dart

@@ -147,6 +147,18 @@ void main() {
 
     test('to DM conversation', () {
       return FakeApiConnection.with_((connection) async {
+        await checkSendMessage(connection,
+          destination: DmDestination(userIds: userIds), content: content,
+          expectedBodyFields: {
+            'type': 'direct',
+            'to': jsonEncode(userIds),
+            'content': content,
+          });
+      });
+    });
+
+    test('to DM conversation, with legacy type "private"', () {
+      return FakeApiConnection.with_(zulipFeatureLevel: 173, (connection) async {
         await checkSendMessage(connection,
           destination: DmDestination(userIds: userIds), content: content,
           expectedBodyFields: {