Fix timestamp comparison in BAT file (#8407)

bin/flutter.bat

@@ -65,10 +65,14 @@ GOTO :after_subroutine
   IF NOT EXIST "%stamp_path%" GOTO do_snapshot
   SET /p stamp_value=<"%stamp_path%"
   IF !stamp_value! NEQ !revision! GOTO do_snapshot
-  REM Get modified timestamps
-  FOR %%f IN ("%flutter_tools_dir%\pubspec.yaml") DO SET yamlt=%%~tf
-  FOR %%a IN ("%flutter_tools_dir%\pubspec.lock") DO SET lockt=%%~ta
-  IF !lockt! LSS !yamlt! GOTO do_snapshot
+  REM Compare "last modified" timestamps
+  SET pubspec_yaml_path=%flutter_tools_dir%\pubspec.yaml
+  SET pubspec_lock_path=%flutter_tools_dir%\pubspec.lock
+  FOR %%i IN ("%pubspec_yaml_path%") DO SET yaml_timestamp=%%~ti
+  FOR %%i IN ("%pubspec_lock_path%") DO SET lock_timestamp=%%~ti
+  IF !yaml_timestamp! EQU !lock_timestamp! GOTO do_snapshot
+  FOR /F %%i IN ('DIR /B /O:D "%pubspec_yaml_path%" "%pubspec_lock_path%"') DO SET newer_file=%%i
+  IF "%newer_file%" EQU "pubspec.yaml" GOTO do_snapshot
 
   REM Everything is uptodate - exit subroutine
   EXIT /B