How to reproduce
Fatal error: Class 'Joomla\String\StringHelper' not found in E:\Server\htdocs\35dev\libraries\joomla\string\string.php on line 23
This is the same reason why travis fails on 3.5-dev: https://travis-ci.org/joomla/joomla-cms/builds/62588602
Other failures
The first failure on 3.5-dev was: 1f2cfac
But with a different error:
PHP Fatal error: Call to undefined method Mock_JApplication_c39b9c17::getLanguageFilter() in /home/travis/build/joomla/joomla-cms/libraries/cms/language/multilang.php on line 41
Committed the missing file from when #6600 was manually merged at b9b2cf3