Provide special route names for <current> and <none>

Lets do one conversion each.

+++ b/core/lib/Drupal/Core/PathProcessor/PathProcessorNone.php
@@ -0,0 +1,27 @@
+      return NULL;

Wouldn't the empty string be more logical to return? NULL violates the interface of processOutbound(), which says to return the processed path, and paths must be strings.

(Plus, it is cast to a string anyway when used, so better to do that right away.)

I'm also reviewing this from the perspective of fixing #2335661: Outbound path & route processors must specify cacheability metadata, which will require path processors to indicate the cacheability of their alterations. If one of the path processors returns NULL instead of a string, that'll be more difficult to solve.

1. +++ b/core/lib/Drupal/Core/PathProcessor/PathProcessorCurrent.php
   @@ -0,0 +1,33 @@
   +      debug($request_uri);
   +      debug($current_base_path);
   
   +++ b/core/modules/node/src/Controller/NodeController.php
   @@ -62,6 +62,8 @@ public static function create(ContainerInterface $container) {
   +    debug(\Drupal::url('<current>'));
   

   I think the new tests look good. Just remove these.

2. +++ b/core/lib/Drupal/Core/PathProcessor/PathProcessorNone.php
   @@ -0,0 +1,27 @@
   +class PathProcessorNone implements OutboundPathProcessorInterface {
   ...
   +      return NULL;
   

   Wasn't that supposed to be return '';

1. +++ b/core/core.services.yml
   @@ -819,6 +819,16 @@ services:
   +    arguments: ['@config.factory']
   ...
   +    arguments: ['@config.factory']
   

   Why do these need the config factory?

2. +++ b/core/modules/node/src/Controller/NodeController.php
   @@ -62,6 +62,8 @@ public static function create(ContainerInterface $container) {
   +    debug(\Drupal::url('<current>'));
   

   Still need to kill this

You know, that's easy enough I can just do it and RTBC. I think @dawehner is traveling today.

This will block #2343669: Remove _l() and _url(), so bumping to critical.

Do we have a followup somewhere to remove current_path()?

Opened #2345753: Remove url(current_path()) and url(NULL) with <current> and <none>.

Committed and pushed to 8.x. Thanks!

Wonderful! Now let's do #2346103: Convert url() and l() invocations using current_path() and '' to actually use them.