Don't consider a timestamp "newer" when equal (#24251)

Fixes flutter/flutter#22062

bin/flutter.bat

@@ -89,6 +89,9 @@ GOTO :after_subroutine
   SET pubspec_yaml_path=%flutter_tools_dir%\pubspec.yaml
   SET pubspec_lock_path=%flutter_tools_dir%\pubspec.lock
   FOR /F %%i IN ('DIR /B /O:D "%pubspec_yaml_path%" "%pubspec_lock_path%"') DO SET newer_file=%%i
+  FOR %%i IN (%pubspec_yaml_path%) DO SET pubspec_yaml_timestamp=%%~ti
+  FOR %%i IN (%pubspec_lock_path%) DO SET pubspec_lock_timestamp=%%~ti
+  IF "%pubspec_yaml_timestamp%" == "%pubspec_lock_timestamp%" SET newer_file=""
   IF "%newer_file%" EQU "pubspec.yaml" GOTO do_snapshot
 
   REM Everything is uptodate - exit subroutine