How to fix the «Class Not_Existing_Class does not exist» issue in Magento 2.0.17 / PHP 7.0.27?

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[ReflectionException]
Class Not_Existing_Class does not exist

01

it is a Windows-only Magento 2.0.x bug.
It is fixed in Magento >= 2.1.0.
You can fix in manually in Magento 2.0.x:
https://github.com/magento/magento2/commit/e07ce736