Magento2 - Setup Compile Error - Class Not_Existing_Class does not exist

Update setup/src/Magento/Setup/Module/Di/Code/Reader/ClassesScanner.php

if (preg_match($pattern, $fileItem->getRealPath())) {

to

if (preg_match($pattern, str_replace('\\', '/', $fileItem->getRealPath()))) {

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