cache MemoryBackend has a pointless constructor

Sure, why not!

nice cleanup

The interdiff of #8 and #11 is:

reverted:
--- b/core/modules/system/src/Tests/System/FloodTest.php
+++ a/core/modules/system/src/Tests/System/FloodTest.php
@@ -61,7 +61,7 @@
     $name = 'flood_test_cleanup';

     $request_stack = \Drupal::service('request_stack');
+    $flood = new MemoryBackend($request_stack);
-    $flood = new MemoryBackend();
     // Register expired event.
     $flood->register($name, $window_expired);
     // Verify event is not allowed.

Actually I spotted a couple more, @catch maybe you missed these or you had a reason to exclude them? I remove the Drupal::service() call too (see interdiff) because it was unused.

Whoops reverted #14 too(same interdiff as #14), should have read #11 closer.

Right, this is just about cache backends, let's be clear about it.

We've also got to fix

    $memoryCounterBackend = new MemoryCounterBackend('default');

Since that backend extends MemoryBackend.

Also I think we need a change records for this.

Sorry missed that thanks @alexpott. Checked for other extends and I think we got them all now.

Added a change record. Feel free to modify it.
https://www.drupal.org/node/2783813

Checked for more cases and quite sure this is back to RTBC, please let me know if my self-RTBC has gotten out of control and I've went too far!

Committed af1abd8 and pushed to 8.3.x. Thanks!