Change record status: 
Introduced in branch: 

In order to support a consistent and comprehensible API for module developers to create links using the Drupal 8.0.x routing API, code that was forward-ported from Drupal 7 has been retired, the use of route name and parameters required in almost all cases and "system path" is not supported except as a temporary value entered into forms by end users that is converted during save.

This change introduces 2 new static factory methods on the \Drupal\Core\Url class:

Url::fromRoute($route_name, $route_parameters, $options)
Url::fromUri($uri, $options)

See api page Url::fromUri() for documentation of arguments.

This change also changes \Drupal\Core\Utility\LinkGenerator so that there is a single generate() method that accepts a Url value object, instead of having multiple methods to generate a link.

The renamed _url() and _l() are deprecated and *will* be removed before 8.0.0 is released.

Drupal 7:

// Internal path.
$internal_link = l(t('Book admin'), 'admin/structure/book');

// External Uri.
$external_link = l(t('External link'), '', array('external' => TRUE));

Drupal 8:

// Internal path (defined by a route in Drupal 8).
use Drupal\Core\Url;
$url = Url::fromRoute('book.admin');
$internal_link = \Drupal::l(t('Book admin'), $url);

// External Uri.
use Drupal\Core\Url;
$url = Url::fromUri('');
$external_link = \Drupal::l(t('External link'), $url);


\Drupal::l has also been deprecated (as of Drupal 8.0.0). Use \Drupal\Core\Link instead.

use Drupal\Core\Link;
use Drupal\Core\Url;
$url = Url::fromRoute('book.admin');
$internal_link = Link::fromTextAndUrl(t('Book admin'), $url)->toString();

Similar for the external uri.

Module developers
Updates Done (doc team, etc.)
Online documentation: 
Not done
Theming guide: 
Not done
Module developer documentation: 
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Examples project: 
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Coder Review: 
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Coder Upgrade: 
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Other updates done