channel: Make channelPostPolicy optional, because deprecated

Author: chrisbobbe

lib/api/model/model.dart

@@ -649,7 +649,7 @@ class ZulipStream {
   bool historyPublicToSubscribers;
   int? messageRetentionDays;
   @JsonKey(name: 'stream_post_policy')
-  ChannelPostPolicy channelPostPolicy;
+  ChannelPostPolicy? channelPostPolicy; // TODO(server-10) remove
   // final bool isAnnouncementOnly; // deprecated for `channelPostPolicy`; ignore
 
   GroupSettingValue? canAddSubscribersGroup; // TODO(server-10)

lib/api/model/model.g.dart

@@ -198,21 +198,25 @@ mixin ChannelStore on UserStore {
     if (inChannel.canSendMessageGroup != null) {
       return selfHasPermissionForGroupSetting(inChannel.canSendMessageGroup!,
                GroupSettingType.stream, 'can_send_message_group');
-    } else {
+    } else if (inChannel.channelPostPolicy != null) {
       return _selfPassesLegacyChannelPostPolicy(inChannel: inChannel, atDate: byDate);
+    } else {
+      assert(false); // TODO(log)
+      return true;
     }
   }
 
   bool _selfPassesLegacyChannelPostPolicy({
     required ZulipStream inChannel,
     required DateTime atDate,
   }) {
+    assert(inChannel.channelPostPolicy != null);
     final role = selfUser.role;
     // We let the users with [unknown] role to send the message, then the server
     // will decide to accept it or not based on its actual role.
     if (role == UserRole.unknown) return true;
 
-    switch (inChannel.channelPostPolicy) {
+    switch (inChannel.channelPostPolicy!) {
       case ChannelPostPolicy.any:             return true;
       case ChannelPostPolicy.fullMembers:     {
         if (!role.isAtLeast(UserRole.member)) return false;