Issue #3425337 by mondrake: Fix root namespace classes DebugClassLoader forward compatibility warnings

Revert "Issue #3425337 by mondrake: Fix root namespace classes DebugClassLoader forward compatibility warnings"

This reverts commit ea9cc753.

Issue #3425337 by mondrake: Fix root namespace classes DebugClassLoader forward compatibility warnings

Revert "Issue #3425337 by mondrake: Fix root namespace classes DebugClassLoader forward compatibility warnings"

This reverts commit ff7c5de3.

Issue #3425337 by mondrake: Fix root namespace classes DebugClassLoader forward compatibility warnings

Issue #2665992 by alexpott, klausi, heykarthikwithu, xjm, tstoeckler: @file is not required for classes, interfaces and traits

Issue #2502621 by Pravin Ajaaz, hussainweb, Peacog, jhodgdon, ivanjaros: Replace implement notes with inheritdoc tag

Issue #2505701 by pwolanin, YesCT, akalata: Document SafeMarkup::set and Use htmlspecialchars() directly in Attribute() so we don't bloat the list of safe strings

Issue #2457887 by prateekMehta, stefan.r, rpayanm, alexpott: Use Utility\SafeMarkup class instead of Utility\String for placeholder(), checkPlain(),format() functions

Issue #2325517 by lauriii, Alienpruts, Tom Verhaeghe, joelpittet, rteijeiro | Cottser: Add methods for adding/removing attributes (not classes) on Attribute objects.

Issue #2336355 by alexpott, Cottser: Fixed Refactor Attribute rendering so that class= is not printed.

Issue #2285451 by joelpittet, davidhernandez, cilefen, mdrummond, mortendk: Create addClass() and removeClass() methods on Attribute object for merging css class names.

Issue #2227869 by joelpittet: Remove magic printing from Attribute class to make Twig 'without' filter work for attributes.

Issue #2212309 by joelpittet: Drupal\Core\Template and twig.engine docs, coding standards, and unused code cleanup.

Issue #2100329 by cosmicdreams: Use \Drupal instead of Drupal to make IDEs and static code analyse tools happy.

Issue #2006282 by joelpittet: Refactor Attribute classes - Cleanup, Security, and Readability and minor performance.

Issue #2003568 by dawehner, jhedstrom: Convert tags,attributes, diff and url validation unit tests to phpunit.

Issue #1805776 by psynaptic, Eric_A, rbayliss: Fixed Fatal error: Class 'Drupal\Core\Template\ArrayIterator' not found in core/lib/Drupal/Core/Template/Attribute.php.

- Patch #1290694 by hefox, tlattimore, c4rl, chx, tim.plunkett, tstoeckler, msonnabaum: provide consistency for attributes and classes arrays provided by template_preprocess().