Generate constructors map as static

bin/tdlib_gen.dart

@@ -240,13 +240,9 @@ abstract class TdBase {
     return 'td::TdBase()';
   }
 
-  static TdBase? fromJson(Map<String, dynamic>? json) {
-    if (json == null) {
-      return null;
-    }
-    final type = json['@type'] as String;
-    final constructors = {
+  static final constructors = {
 """;
+
   for (final o in scheme.objects) {
     final normName = o.name.pascalCase;
     result += '''
@@ -254,12 +250,18 @@ abstract class TdBase {
 ''';
   }
 
-  result += '''
+  result += """
   };
+
+  static TdBase? fromJson(Map<String, dynamic>? json) {
+    if (json == null) {
+      return null;
+    }
+    final type = json['@type'] as String;
     return constructors[type]!(json);
   }
 }
-''';
+""";
   return result;
 }
 String makeAbstractFile(TlSchema scheme) {