refactor: make constructors const

Author: kosukesaigusa

packages/from_json_generator/lib/src/configs/build_yaml_config.dart

@@ -1,7 +1,7 @@
 /// Configuration values defined in build.yaml.
 class BuildYamlConfig {
   /// Creates a [BuildYamlConfig].
-  BuildYamlConfig({required this.convertSnakeToCamel});
+  const BuildYamlConfig({required this.convertSnakeToCamel});
 
   /// Decode the options from a build.yaml.
   factory BuildYamlConfig.fromBuildYaml(Map<String, dynamic> yaml) =>

packages/from_json_generator/lib/src/from_json_generator.dart

@@ -12,7 +12,7 @@ import 'utils/dart_object_util.dart';
 /// A generator for [FromJson] annotation.
 class FromJsonGenerator extends GeneratorForAnnotation<FromJson> {
   /// Creates a new instance of [FromJsonGenerator].
-  FromJsonGenerator(this._buildYamlConfig);
+  const FromJsonGenerator(this._buildYamlConfig);
 
   /// A [BuildYamlConfig] instance.
   final BuildYamlConfig _buildYamlConfig;