Refactor atom.dart to make code cleaner (#854)

* Update atom.dart

* chore: add a version change

---------

Co-authored-by: Pavan Podila <pavanpodila@users.noreply.github.com>
Co-authored-by: Minsu Lee <amond@amond.net>

mobx/CHANGELOG.md

@@ -1,7 +1,8 @@
-## 2.1.2 - 2.1.3
+## 2.1.2 - 2.1.3+1
 
 - Fix tests in dart 2.19 - [@amondnet](https://github.com/amondnet)
 - Dart formatting fixes
+- Refactor `atom.dart` to make code cleaner by [@fzyzcjy](https://github.com/fzyzcjy) in [#854](https://github.com/mobxjs/mobx.dart/pull/854)
 
 ## 2.1.1
 

mobx/lib/src/core/atom.dart

@@ -96,11 +96,7 @@ class Atom {
  _addListener(_ListenerKind.onBecomeUnobserved, fn);
 
  void Function() _addListener(_ListenerKind kind, void Function() fn) {
- if (_observationListeners[kind] == null) {
- _observationListeners[kind] = {}..add(fn);
- } else {
- _observationListeners[kind]!.add(fn);
- }
+ (_observationListeners[kind] ??= {}).add(fn);
 
  return () {
  final listeners = _observationListeners[kind];

mobx/lib/version.dart

@@ -1,4 +1,4 @@
 // Generated via set_version.dart. !!!DO NOT MODIFY BY HAND!!!
 
 /// The current version as per `pubspec.yaml`.
-const version = '2.1.3';
+const version = '2.1.3+1';

mobx/pubspec.yaml

@@ -1,5 +1,5 @@
 name: mobx
-version: 2.1.3
+version: 2.1.3+1
 description: "MobX is a library for reactively managing the state of your applications. Use the power of observables, actions, and reactions to supercharge your Dart and Flutter apps."
 
 homepage: https://github.com/mobxjs/mobx.dart