diff --git a/core/tests/Drupal/Tests/Core/Extension/ExtensionDiscoveryTest.php b/core/tests/Drupal/Tests/Core/Extension/ExtensionDiscoveryTest.php
index bcc6dc8..919603d 100644
--- a/core/tests/Drupal/Tests/Core/Extension/ExtensionDiscoveryTest.php
+++ b/core/tests/Drupal/Tests/Core/Extension/ExtensionDiscoveryTest.php
@@ -1,10 +1,5 @@
 <?php
 
-/**
- * @file
- * Contains \Drupal\Tests\Core\Extension\ExtensionDiscoveryTest.
- */
-
 namespace Drupal\Tests\Core\Extension;
 
 use Drupal\Core\Extension\ExtensionDiscovery;
@@ -30,104 +25,80 @@ public function providerModule() {
 core: 8.x
 EOS;
     return [
-      'sites' => [
-        'default' => [
-          'modules' => [
-            'module_a' => [
-              'module_a.info.yml' => $info_a
+      [
+        'sites' => [
+          'default' => [
+            'modules' => [
+              'module_a' => [
+                'module_a.info.yml' => $info_a
+              ],
             ],
-          ]
-        ]
+          ],
+        ],
       ],
     ];
   }
 
   /**
-   * Check that the cache is loaded and used.
+   * Check that the cache uses different keys for different root directories.
    *
    * @covers ::scan
    *
    * @dataProvider providerModule
    */
-  public function testScanCache($filesystem) {
-    vfsStream::setup('test_a');
-    vfsStream::create($filesystem);
-
-    $expected = ['test_scanned_directory'];
-
-    // Set up a mocked ExtensionDiscovery object so we can mock the
-    // scanDirectory() method.
-    $mock_extension_discovery = $this->getMockBuilder(ExtensionDiscovery::class)
-      ->setConstructorArgs([vfsStream::url('test_a'), TRUE, ['no/profile'], 'sites/default'])
-      ->setMethods(['scanDirectory'])
-      ->getMock();
-    // The scanDirectory() method gets called 4 times while traversing the file
-    // system once, one each for ORIGIN_CORE, ORIGIN_SITE_ALL, etc.
-    $mock_extension_discovery->expects($this->exactly(4))
-      ->method('scanDirectory')
-      ->willReturn($expected);
-    // Use up our 4 calls to scanDirectory().
-    $mock_extension_discovery->scan('module', FALSE);
-
-    // Subsequent scans should not call scanDirectory().
-    $mock_extension_discovery->expects($this->exactly(0))
-      ->method('scanDirectory')
-      ->willReturn($expected);
-    $mock_extension_discovery->scan('module', FALSE);
-  }
-
-  /**
-   * Check that the cache is invalidated when needed.
-   *
-   * @covers ::scan
-   *
-   * @dataProvider providerModule
-   */
-  public function testScanCacheInvalidate($filesystem) {
+  public function testScanCacheKeys($filesystem) {
     vfsStream::setup('root');
 
     // Create the same filesystem in two different directories.
     vfsStream::create($filesystem, vfsStream::newDirectory('test1'));
     vfsStream::create($filesystem, vfsStream::newDirectory('test2'));
 
-    $expected = ['test_scanned_directory'];
-
-    // Set up a mocked ExtensionDiscovery object so we can mock the
-    // scanDirectory() method. We'll use the test1 directory first.
-    $test1_mock_extension_discovery = $this->getMockBuilder(ExtensionDiscovery::class)
-      ->setConstructorArgs([vfsStream::url('root/test1'), TRUE, ['no/profile'], 'sites/default'])
-      ->setMethods(['scanDirectory'])
-      ->getMock();
-    // The scanDirectory() method gets called 4 times while traversing the file
-    // system once, one each for ORIGIN_CORE, ORIGIN_SITE_ALL, etc.
-    $test1_mock_extension_discovery->expects($this->exactly(4))
-      ->method('scanDirectory')
-      ->willReturn($expected);
-
-    // Use up our 4 calls to scanDirectory().
-    $test1_mock_extension_discovery->scan('module', FALSE);
-
-    // Verify that when we mock ExtensionDiscovery again, the static is still
-    // present.
-    $test1_mock_extension_discovery = $this->getMockBuilder(ExtensionDiscovery::class)
-      ->setConstructorArgs([vfsStream::url('root/test1'), TRUE, ['no/profile'], 'sites/default'])
-      ->setMethods(['scanDirectory'])
-      ->getMock();
-    // scanDirectory() should be called 0 times if the cache is still there.
-    $test1_mock_extension_discovery->expects($this->exactly(0))
-      ->method('scanDirectory')
-      ->willReturn($expected);
-    $test1_mock_extension_discovery->scan('module', FALSE);
-
-    // Now use test2 in order to check that the cache was invalidated.
-    $test2_mock_extension_discovery = $this->getMockBuilder(ExtensionDiscovery::class)
-      ->setConstructorArgs([vfsStream::url('root/test2'), TRUE, ['no/profile'], 'sites/default'])
-      ->setMethods(['scanDirectory'])
-      ->getMock();
-    $test2_mock_extension_discovery->expects($this->exactly(4))
-      ->method('scanDirectory')
-      ->willReturn($expected);
-    $test2_mock_extension_discovery->scan('module', FALSE);
+    $root_1 = vfsStream::url('root/test1');
+    $root_2 = vfsStream::url('root/test2');
+
+    // Create an ExtensionDiscovery with $root_1.
+    $extension_discovery = new ExtensionDiscovery($root_1, TRUE, ['no/profile'], 'sites/default');
+
+    // Assert that the cache does not contain either key we're looking for until
+    // after we've performed a scan.
+    $this->assertArrayNotHasKey($root_1, $this->getDiscoveredCache($extension_discovery));
+    $this->assertArrayNotHasKey($root_2, $this->getDiscoveredCache($extension_discovery));
+
+    // Perform the scan.
+    $extension_discovery->scan('module');
+
+    // Assert that we have $root_1 as a key, but not $root_2 yet.
+    $this->assertArrayHasKey($root_1, $this->getDiscoveredCache($extension_discovery));
+    $this->assertArrayNotHasKey($root_2, $this->getDiscoveredCache($extension_discovery));
+
+    // Re-instantiate ExtensionDiscovery with a different root.
+    $extension_discovery = new ExtensionDiscovery($root_2, TRUE, ['no/profile'], 'sites/default');
+
+    // Since the cache is static, it will still only have $root_1.
+    $this->assertArrayHasKey($root_1, $this->getDiscoveredCache($extension_discovery));
+    $this->assertArrayNotHasKey($root_2, $this->getDiscoveredCache($extension_discovery));
+
+    // Perform the scan.
+    $extension_discovery->scan('module');
+
+    // Now we have both keys in the cache array.
+    $this->assertArrayHasKey($root_1, $this->getDiscoveredCache($extension_discovery));
+    $this->assertArrayHasKey($root_2, $this->getDiscoveredCache($extension_discovery));
+  }
+
+  /**
+   * Helper method to get the cache array from an ExtensionDiscovery object.
+   *
+   * @param ExtensionDiscovery $extension_discovery
+   *   An ExtensionDiscovery object.
+   *
+   * @return array[]
+   *   The files array.
+   */
+  protected function getDiscoveredCache(ExtensionDiscovery $extension_discovery) {
+    $ref_cache = new \ReflectionProperty(ExtensionDiscovery::class, 'files');
+    $ref_cache->setAccessible(TRUE);
+    return $ref_cache->getValue($extension_discovery);
   }
 
 }