kitchen sink: make all test collections const

Author: kevmoo

json_serializable/lib/src/constants.dart

@@ -2,5 +2,5 @@
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
-final toJsonMapVarName = 'val';
-final toJsonMapHelperName = 'writeNotNull';
+const toJsonMapVarName = 'val';
+const toJsonMapHelperName = 'writeNotNull';

json_serializable/test/kitchen_sink_test.dart

@@ -240,15 +240,15 @@ final _invalidValues = const {
   r'$string': true,
 };
 
-final _excludeIfNullKeys = [
+final _excludeIfNullKeys = const [
   'dateTime',
   'iterable',
   'dateTimeList',
   'crazyComplex',
   toJsonMapVarName
 ];
 
-final _expectedOrder = [
+final _expectedOrder = const [
   'no-42',
   'dateTime',
   'iterable',