Drupal 8.0.0 beta 12. More info: https://www.drupal.org/node/2514176

core/vendor/symfony/debug/README.md

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+Debug Component
+===============
+
+Debug provides tools to make debugging easier.
+
+Enabling all debug tools is as easy as calling the `enable()` method on the
+main `Debug` class:
+
+```php
+use Symfony\Component\Debug\Debug;
+
+Debug::enable();
+```
+
+You can also use the tools individually:
+
+```php
+use Symfony\Component\Debug\ErrorHandler;
+use Symfony\Component\Debug\ExceptionHandler;
+
+if ('cli' !== php_sapi_name()) {
+    ini_set('display_errors', 0);
+    ExceptionHandler::register();
+} elseif (!ini_get('log_errors') || ini_get('error_log')) {
+    ini_set('display_errors', 1);
+}
+ErrorHandler::register();
+```
+
+Note that the `Debug::enable()` call also registers the debug class loader
+from the Symfony ClassLoader component when available.
+
+This component can optionally take advantage of the features of the HttpKernel
+and HttpFoundation components.
+
+Resources
+---------
+
+You can run the unit tests with the following command:
+
+    $ cd path/to/Symfony/Component/Debug/
+    $ composer install
+    $ phpunit