class Google_Service not found - vendor/autoload is called

Question

Per the docs, I have required google/apiclient:^2.12.1, and have the following code in my project:

require_once( $_SERVER['COMP_ROOT'] . "/vendor/autoload.php" );

$default_scope_set = [
                        Google_Service_Plus::PLUS_LOGIN,
                        Google_Service_Plus::PLUS_ME,
                        Google_Service_Plus::USERINFO_EMAIL,
                     ];

When I try to load up my page, this error is thrown on the line "Google_Service_Plus::PLUS_LOGIN":

Error: Class 'Google_Service' not found in home/httpd/idxv3/lib/Google/Service/Plus.php:32
Stack trace:
#0 /home/httpd/gears/lib/AutoLoader.php(127): include_once()
#1 [internal function]: AutoLoader::load()
#2 [internal function]: spl_autoload_call()
#3 /home/httpd/idxv3/vendor/google/apiclient-services/autoload.php(31): class_exists()
#4 [internal function]: {closure}()
#5 /home/httpd/idxv3/lib/GoogleAuthenticator.php(45): spl_autoload_call()
#6 /home/httpd/idxv3/app/controllers/web/WebAdminController.php(943): GoogleAuthenticator->__construct()
#7 /home/httpd/gears/lib/Dispatcher.php(60): WebAdminController->login()
#8 /home/httpd/gears/lib/Dispatcher.php(916): Dispatcher::call()
#9 /home/httpd/idxv3/public/dispatch.php(10): Dispatcher::dispatch()
#10 {main}

Most likely related, somewhat before, this warning is in my logs:

PHP Warning:  Class 'Google_Service' not found in /home/httpd/idxv3/vendor/google/apiclient-services/autoload.php on line 21

I'm at a loss. Been beating on this all day.