[BUG][DependencyInjection] Syntax error in PHP dumper

When creating the cache, a syntax error is still present since the issue #9807. The #9812 fix does not solve the problem.

PHP Parse error:  syntax error, unexpected '\' (T_NS_SEPARATOR), expecting identifier (T_STRING) in ./app/cache/dev/appDevDebugPr
ojectContainer.php

In my case, the line:

$g = \Doctrine\\ORM\\EntityManager::create($this->get('doctrine.dbal.default_connection'), $f);

should be formatted like this:

$g = \Doctrine\ORM\EntityManager::create($this->get('doctrine.dbal.default_connection'), $f);

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