I use the Joomla! Logger in the installation script file of a custom Joomla! component (Visforms), in order to write a log file.
I use the following Code to include the Joomla! Logger and extend the JLogLoggerFormattedtext

...
jimport('joomla.log.log');
class JLogLoggerFormattedtextVisforms extends JLogLoggerFormattedtext
{
	protected $priorities = array(
		JLog::EMERGENCY => 'EMG',
		JLog::ALERT => 'ALT',
		JLog::CRITICAL => 'CRI',
		JLog::ERROR => 'ERR',
		JLog::WARNING => 'WRN',
		JLog::NOTICE => 'NTC',
		JLog::INFO => 'INF',
		JLog::DEBUG => 'DBG'
       );
	
	public function addEntry(JLogEntry $entry)
	{
		try {
		    @parent::addEntry($entry);
		} catch (RuntimeException $e) {
			//prevent installation from being aborted as failed, if we cannot write the log file
		}
	}
}

class com_visformsInstallerScript
{
	...
}

Installing the component on Joomla! 3.8.0 rc1 fails whereas the exact same installation works on Joomla! 3.7.5 properly without any problems

Steps to reproduce the issue

Visforms is available from JED through "Install from Web", if you want to reproduce the problem.
Install on Joomla! 3.7.5 works without problems. (Message: Message "Installation of the component was successful.")
Install on Joomla! 3.8.0 rc1 throws the error message:
Unable to create a Logger instance: Joomla\CMS\Log\Logger\FormattedtextvisformsLogger

Expected result

On Joomla! 3.8.0 rc1 although: Message "Installation of the component was successful."

Actual result

error message:
Unable to create a Logger instance: Joomla\CMS\Log\Logger\FormattedtextvisformsLogger

System information (as much as possible)

php 7.1.7 on local XAMPP 7.1.7

Additional comments

I actually think I have extended the Joomla! Logger in the intended way and that the code posted above should still work with Joomla! 3.8.0, so this is a backward compatibility problem with the installer.
I think after moving the Logger to the librairies/src folder with Joomla! 3.8.0 it is now necessary to use the proper namespace, which should be implemented by the Joomla! installer.
Please let me know, if I'm wrong.

Regards,
Aicha