recent_senders [nfc]: Verify that message ID lists are sorted

The data structure and its helpers rely on message IDs being sorted,
otherwise its behavior is unpredictable.

This will provide useful diagnostic information when that happens. This
is NFC because no change is made live, i.e. no additional sorting.

Author: PIG208

lib/model/recent_senders.dart

@@ -137,14 +137,19 @@ class MessageIdTracker {
   ///
   /// [newIds] should be sorted ascending.
   void addAll(QueueList<int> newIds) {
+    assert(isSortedWithoutDuplicates(newIds));
     if (ids.isEmpty) {
       ids = newIds;
       return;
     }
     ids = setUnion(ids, newIds);
   }
 
+  /// Remove message IDs found in [idsToRemove] from the tracker list.
+  ///
+  /// [idsToRemove] should be sorted ascending.
   void removeAll(List<int> idsToRemove) {
+    assert(isSortedWithoutDuplicates(idsToRemove));
     ids.removeWhere((id) => binarySearch(idsToRemove, id) != -1);
   }