autocomplete: Sort user-mention autocomplete results

Fixes: zulip#228

Author: sm-sayedi

lib/model/autocomplete.dart

@@ -1,3 +1,4 @@
+import 'package:collection/collection.dart';
 import 'package:flutter/foundation.dart';
 import 'package:flutter/services.dart';
 
@@ -266,11 +267,12 @@ class MentionAutocompleteView extends ChangeNotifier {
 
         final User user = iterator.current;
         if (query.testUser(user, store.autocompleteViewManager.autocompleteDataCache)) {
-          results.add(UserMentionAutocompleteResult(userId: user.userId));
+          results.add(UserMentionAutocompleteResult(userId: user.userId, fullName: user.fullName));
         }
       }
     }
-    return results; // TODO(#228) sort for most relevant first
+    return results.sortedBy((result) =>
+      (result as UserMentionAutocompleteResult).fullName.toLowerCase()); // TODO(#228) sort for most relevant first
   }
 }
 
@@ -344,9 +346,10 @@ class AutocompleteDataCache {
 sealed class MentionAutocompleteResult {}
 
 class UserMentionAutocompleteResult extends MentionAutocompleteResult {
-  UserMentionAutocompleteResult({required this.userId});
+  UserMentionAutocompleteResult({required this.userId, required this.fullName});
 
   final int userId;
+  final String fullName;
 }
 
 // TODO(#233): // class UserGroupMentionAutocompleteResult extends MentionAutocompleteResult {