Update to drupal 8.0.0-rc1. For more information, see https://www.drupal.org/node/2582663

vendor/symfony/css-selector/README.md

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+CssSelector Component
+=====================
+
+CssSelector converts CSS selectors to XPath expressions.
+
+The component only goal is to convert CSS selectors to their XPath
+equivalents:
+
+```php
+use Symfony\Component\CssSelector\CssSelector;
+
+print CssSelector::toXPath('div.item > h4 > a');
+```
+
+HTML and XML are different
+--------------------------
+
+The `CssSelector` component comes with an `HTML` extension which is enabled by
+default. If you need to use this component with `XML` documents, you have to
+disable this `HTML` extension. That's because, `HTML` tag & attribute names
+are always lower-cased, but case-sensitive in `XML`:
+
+```php
+// disable `HTML` extension:
+CssSelector::disableHtmlExtension();
+
+// re-enable `HTML` extension:
+CssSelector::enableHtmlExtension();
+```
+
+When the `HTML` extension is enabled, tag names are lower-cased, attribute
+names are lower-cased, the following extra pseudo-classes are supported:
+`checked`, `link`, `disabled`, `enabled`, `selected`, `invalid`, `hover`,
+`visited`, and the `lang()` function is also added.
+
+Resources
+---------
+
+This component is a port of the Python cssselect library
+[v0.7.1](https://github.com/SimonSapin/cssselect/releases/tag/v0.7.1),
+which is distributed under the BSD license.
+
+You can run the unit tests with the following command:
+
+    $ cd path/to/Symfony/Component/CssSelector/
+    $ composer install
+    $ phpunit