Generate constructors map as static

bin/tdlib_gen.dart

@@ -240,26 +240,28 @@ abstract class TdBase {
     return 'td::TdBase()';
   }
 
+  static final constructors = {
+""";
+
+  for (final o in scheme.objects) {
+    final normName = o.name.pascalCase;
+    result += '''
+    '${o.name}': (json) => o.$normName.fromJson(json),
+''';
+  }
+
+  result += """
+  };
+
   static TdBase? fromJson(Map<String, dynamic>? json) {
     if (json == null) {
       return null;
     }
     final type = json['@type'] as String;
-    final constructors = {
-""";
-  for (final o in scheme.objects) {
-    final normName = o.name.pascalCase;
-    result += '''
-      '${o.name}': (json) => o.$normName.fromJson(json),
-''';
-  }
-
-  result += '''
-    };
     return constructors[type]!(json);
   }
 }
-''';
+""";
   return result;
 }
 String makeAbstractFile(TlSchema scheme) {