diff -u b/core/tests/Drupal/Tests/Core/Cache/InconsistentBackendTest.php b/core/tests/Drupal/Tests/Core/Cache/InconsistentBackendTest.php
--- b/core/tests/Drupal/Tests/Core/Cache/InconsistentBackendTest.php
+++ b/core/tests/Drupal/Tests/Core/Cache/InconsistentBackendTest.php
@@ -45,21 +45,24 @@
   }
 
   public function testFallThroughToConsistentCache() {
-    // TODO: Refactor to use mocks for at least inconsistent cache backend so
-    // that we can assert that ::set gets called n times.
     $state = $this->getMock("Drupal\Core\KeyValueStore\StateInterface");
     $state->expects($this->once())
           ->method("get")
           ->will($this->returnValue(time() + 99999));
 
+    $inconsistent_cache = $this->getMock("Drupal\Core\Cache\MemoryBackend",
+      array("set"), array("foo"));
+    $inconsistent_cache->expects($this->exactly(2))
+                       ->method("set");
+
     $inconsistent_backend = new ChainedConsistentAndInconsistentBackend(
       $this->consistent_cache,
-      $this->inconsistent_cache,
+      $inconsistent_cache,
       $this->bin,
       $state
     );
 
-    $this->inconsistent_cache->set("foo", "bar");
+    $inconsistent_cache->set("foo", "bar");
     $this->consistent_cache->set("foo", "baz");
     $this->assertEquals("baz", $inconsistent_backend->get("foo")->data);
   }