Issue #2352949 by Wim Leers, akalata, Jaesin, lauriii, dww, alexpott, Berdir, dawehner, tstoeckler, xjm, andypost: Deprecate using Classy as the default theme for the 'testing' profile
// Require a default theme to be specified at this point.
if(!isset($this->defaultTheme)){
// For backwards compatibility, tests using the 'testing' install profile
// on Drupal 8 automatically get 'classy' set, and other profiles use
// 'stark'.
@trigger_error('Drupal\Tests\BrowserTestBase::$defaultTheme is required in drupal:9.0.0 when using an install profile that does not set a default theme. See https://www.drupal.org/node/2352949, which includes recommendations on which theme to use.',E_USER_DEPRECATED);