Flutter - Error: The language version override has to be the same in the library and its part(s)

I was update my Flutter SDK version from dev channel (1.14.5) to stable channel (2.0.1). My codes was works perfectly but now i get this error. I think the problem is at slide_countdown_clock but im not sure.

    lib/Screens/sets4_widget/Countdown.dart:7:6: Error: The language version override has to be the same in the library and its part(s).
part 'package:slide_countdown_clock/clip_digit.dart';
     ^
lib/Screens/sets4_widget/Countdown.dart:8:6: Error: The language version override has to be the same in the library and its part(s).
part 'package:slide_countdown_clock/digit.dart';
     ^
lib/Screens/sets4_widget/Countdown.dart:9:6: Error: The language version override has to be the same in the library and its part(s).
part 'package:slide_countdown_clock/slide_direction.dart';
     ^


FAILURE: Build failed with an exception.

* Where:
Script 'C:\flutter\packages\flutter_tools\gradle\flutter.gradle' line: 991

* What went wrong:
Execution failed for task ':app:compileFlutterBuildDebug'.
> Process 'command 'C:\flutter\bin\flutter.bat'' finished with non-zero exit value 1

* Try:
Run with --stacktrace option to get the stack trace. Run with --info or --debug option to get more log output. Run with --scan to get full insights.

* Get more help at https://help.gradle.org

BUILD FAILED in 13s
Exception: Gradle task assembleDebug failed with exit code 1
     ^

I also share the lines of code that the error pointed to me below.

part 'package:slide_countdown_clock/clip_digit.dart';
part 'package:slide_countdown_clock/digit.dart';
part 'package:slide_countdown_clock/slide_direction.dart';

those parts are give me error. I tried flutter clean > flutter upgrade > flutter pub upgrade but still no result. Can anyone please help?