api: Add + operator on ServerEmojiData

This operator which right now is only intended for test usage, makes it
easy to combine two emoji data objects with the control of which one
overrides which.

Author: sm-sayedi

lib/api/route/realm.dart

@@ -1,3 +1,4 @@
+import 'package:flutter/foundation.dart';
 import 'package:http/http.dart' as http;
 import 'package:json_annotation/json_annotation.dart';
 
@@ -137,6 +138,16 @@ class ServerEmojiData {
 
   ServerEmojiData({required this.codeToNames});
 
+  /// Returns this emoji data merged with [other].
+  ///
+  /// Returns a new `ServerEmojiData` object combining [codeToNames] of [this]
+  /// and [other]. If there is any duplicate code key, its resulting names value
+  /// will be from [other.codeToNames].
+  @visibleForTesting
+  ServerEmojiData operator +(ServerEmojiData other) {
+    return ServerEmojiData(codeToNames: {...codeToNames, ...other.codeToNames});
+  }
+
   factory ServerEmojiData.fromJson(Map<String, dynamic> json) =>
     _$ServerEmojiDataFromJson(json);