copy constants - cannot publish otherwise (#800)

json_serializable/lib/src/constants.dart

@@ -2,8 +2,6 @@
 // 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.
 
-// @dart=2.12
-
 /// Name used for closure argument when generating calls to `map`.
 const closureArg = 'e';
 

json_serializable/test/src/_json_serializable_test_input.dart

@@ -7,11 +7,12 @@
 import 'dart:collection';
 
 import 'package:json_annotation/json_annotation.dart';
-import 'package:json_serializable/src/constants.dart';
 
 // ignore: import_of_legacy_library_into_null_safe
 import 'package:source_gen_test/annotations.dart';
 
+part 'constants_copy.dart';
+
 part 'checked_test_input.dart';
 
 part 'core_subclass_type_input.dart';

json_serializable/test/src/constants_copy.dart

@@ -0,0 +1,23 @@
+// Copyright (c) 2021, the Dart project authors.  Please see the AUTHORS file
+// 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.
+
+// @dart=2.12
+
+part of '_json_serializable_test_input.dart';
+
+// TODO: remove this and return link to lib/src/constants.dart once this
+//  package runs with full null safety
+
+/// Name used for closure argument when generating calls to `map`.
+const closureArg = 'e';
+
+const generatedLocalVarName = 'val';
+const toJsonMapHelperName = 'writeNotNull';
+
+const converterOrKeyInstructions = r'''
+* Use `JsonConverter`
+  https://pub.dev/documentation/json_annotation/latest/json_annotation/JsonConverter-class.html
+* Use `JsonKey` fields `fromJson` and `toJson`
+  https://pub.dev/documentation/json_annotation/latest/json_annotation/JsonKey/fromJson.html
+  https://pub.dev/documentation/json_annotation/latest/json_annotation/JsonKey/toJson.html''';