Seems like there is some syntax error left while you were trying to increase the memory limit by editing the wp-config.php file .
Please check your wp-config.php file for this .
It isn't likely that the error is being caused by /wp-includes/default-constants.php unless you can verify that the file has actually been altered at some point in the past. In that case, download the WordPress package, replace just the default-constants.php file with a new copy and test the result.
Otherwise, temporarily switch back to the default WordPress theme and see if the symptom changes.