message [nfc]: Move sendMessage from PerAccountStore

MessageStore stands out as a better home for sendMessage, which was on
PerAccountStore before we extracted all the separate stores.  This will
make it more natural to support outbox/local echoing.

Author: PIG208

lib/model/message.dart

@@ -1,10 +1,14 @@
 import 'dart:convert';
 
+import '../api/core.dart';
 import '../api/model/events.dart';
 import '../api/model/model.dart';
+import '../api/route/messages.dart';
 import '../log.dart';
 import 'message_list.dart';
 
+const _apiSendMessage = sendMessage; // Bit ugly; for alternatives, see: https://chat.zulip.org/#narrow/stream/243-mobile-team/topic/flutter.3A.20PerAccountStore.20methods/near/1545809
+
 /// The portion of [PerAccountStore] for messages and message lists.
 mixin MessageStore {
   /// All known messages, indexed by [Message.id].
@@ -15,6 +19,11 @@ mixin MessageStore {
   void registerMessageList(MessageListView view);
   void unregisterMessageList(MessageListView view);
 
+  Future<void> sendMessage({
+    required MessageDestination destination,
+    required String content,
+  });
+
   /// Reconcile a batch of just-fetched messages with the store,
   /// mutating the list.
   ///
@@ -29,11 +38,13 @@ mixin MessageStore {
 }
 
 class MessageStoreImpl with MessageStore {
-  MessageStoreImpl()
+  MessageStoreImpl({required this.connection})
     // There are no messages in InitialSnapshot, so we don't have
     // a use case for initializing MessageStore with nonempty [messages].
     : messages = {};
 
+  final ApiConnection connection;
+
   @override
   final Map<int, Message> messages;
 
@@ -77,6 +88,17 @@ class MessageStoreImpl with MessageStore {
     //     https://chat.zulip.org/#narrow/channel/243-mobile-team/topic/MessageListView.20lifecycle/near/2086893
   }
 
+  @override
+  Future<void> sendMessage({required MessageDestination destination, required String content}) {
+    // TODO implement outbox; see design at
+    //   https://chat.zulip.org/#narrow/stream/243-mobile-team/topic/.23M3881.20Sending.20outbox.20messages.20is.20fraught.20with.20issues/near/1405739
+    return _apiSendMessage(connection,
+      destination: destination,
+      content: content,
+      readBySender: true,
+    );
+  }
+
   @override
   void reconcileMessages(List<Message> messages) {
     // What to do when some of the just-fetched messages are already known?

lib/model/store.dart

@@ -393,7 +393,7 @@ class PerAccountStore extends ChangeNotifier with EmojiStore, UserStore, Channel
         typingStartedExpiryPeriod: Duration(milliseconds: initialSnapshot.serverTypingStartedExpiryPeriodMilliseconds),
       ),
       channels: channels,
-      messages: MessageStoreImpl(),
+      messages: MessageStoreImpl(connection: connection),
       unreads: Unreads(
         initial: initialSnapshot.unreadMsgs,
         selfUserId: account.userId,
@@ -829,16 +829,10 @@ class PerAccountStore extends ChangeNotifier with EmojiStore, UserStore, Channel
     }
   }
 
+  @override
   Future<void> sendMessage({required MessageDestination destination, required String content}) {
     assert(!_disposed);
-
-    // TODO implement outbox; see design at
-    //   https://chat.zulip.org/#narrow/stream/243-mobile-team/topic/.23M3881.20Sending.20outbox.20messages.20is.20fraught.20with.20issues/near/1405739
-    return _apiSendMessage(connection,
-      destination: destination,
-      content: content,
-      readBySender: true,
-    );
+    return _messages.sendMessage(destination: destination, content: content);
   }
 
   static List<CustomProfileField> _sortCustomProfileFields(List<CustomProfileField> initialCustomProfileFields) {
@@ -860,7 +854,6 @@ class PerAccountStore extends ChangeNotifier with EmojiStore, UserStore, Channel
   String toString() => '${objectRuntimeType(this, 'PerAccountStore')}#${shortHash(this)}';
 }
 
-const _apiSendMessage = sendMessage; // Bit ugly; for alternatives, see: https://chat.zulip.org/#narrow/stream/243-mobile-team/topic/flutter.3A.20PerAccountStore.20methods/near/1545809
 const _tryResolveUrl = tryResolveUrl;
 
 /// Like [Uri.resolve], but on failure return null instead of throwing.