diff --git a/core/modules/simpletest/lib/Drupal/simpletest/DrupalUnitTestBase.php b/core/modules/simpletest/lib/Drupal/simpletest/DrupalUnitTestBase.php
index f241069..18b3ae8 100644
--- a/core/modules/simpletest/lib/Drupal/simpletest/DrupalUnitTestBase.php
+++ b/core/modules/simpletest/lib/Drupal/simpletest/DrupalUnitTestBase.php
@@ -64,7 +64,9 @@
   protected $keyValueFactory;
 
   /**
-   * A list of stream wrappers that have been registered by this test.
+   * A list of stream wrappers that have been registered for this test.
+   *
+   * @see \Drupal\simpletest\DrupalUnitTestBase::registerStreamWrapper()
    *
    * @var array
    */
@@ -141,9 +143,10 @@ protected function setUp() {
 
     // Tests based on DrupalUnitTestBase are entitled to use Drupal's File API.
     $this->registerStreamWrapper('public', 'Drupal\Core\StreamWrapper\PublicStream');
-    // The private and temporary stream wrappers depend on configuration, but if
-    // System module was enabled, they can be registered. The test only needs
-    // to install System module's configuration.
+
+    // The private and temporary stream wrappers depend on the system.file
+    // configuration. If System module was enabled, the test will most likely
+    // install System module's configuration, so we can register them here.
     if (in_array('system', $modules)) {
       $this->registerStreamWrapper('private', 'Drupal\Core\StreamWrapper\PrivateStream');
       $this->registerStreamWrapper('temporary', 'Drupal\Core\StreamWrapper\TemporaryStream');
@@ -152,6 +155,10 @@ protected function setUp() {
 
   protected function tearDown() {
     $this->kernel->shutdown();
+    // Before tearing down the test environment, ensure that no stream wrapper
+    // of this test leaks into the parent environment. Unlike all other global
+    // state variables in Drupal, stream wrappers are a global state construct
+    // of PHP core, which has to be maintained manually.
     foreach ($this->stream_wrappers as $scheme) {
       $this->unregisterStreamWrapper($scheme);
     }
@@ -371,7 +378,12 @@ protected function registerStreamWrapper($scheme, $class, $type = STREAM_WRAPPER
       $this->unregisterStreamWrapper($scheme);
     }
     $this->stream_wrappers[$scheme] = $scheme;
-    stream_wrapper_register($scheme, $class);
+    if (($type & STREAM_WRAPPERS_LOCAL) == STREAM_WRAPPERS_LOCAL) {
+      stream_wrapper_register($scheme, $class);
+    }
+    else {
+      stream_wrapper_register($scheme, $class, STREAM_IS_URL);
+    }
     // @todo Revamp Drupal's stream wrapper API for D8.
     $wrappers = &drupal_static('file_get_stream_wrappers');
     $wrappers[$scheme] = array(
@@ -384,6 +396,9 @@ protected function registerStreamWrapper($scheme, $class, $type = STREAM_WRAPPER
   /**
    * Unregisters a stream wrapper previously registered by this test.
    *
+   * DrupalUnitTestBase::tearDown() automatically cleans up all registered
+   * stream wrappers, so this usually does not have to be called manually.
+   *
    * @param string $scheme
    *   The scheme to unregister.
    */
diff --git a/core/modules/simpletest/lib/Drupal/simpletest/TestBase.php b/core/modules/simpletest/lib/Drupal/simpletest/TestBase.php
index a859d93..3bb6e3f 100644
--- a/core/modules/simpletest/lib/Drupal/simpletest/TestBase.php
+++ b/core/modules/simpletest/lib/Drupal/simpletest/TestBase.php
@@ -963,7 +963,14 @@ protected function prepareEnvironment() {
     // Create and set new configuration directories.
     $this->prepareConfigDirectories();
 
-    // Unregister all Drupal stream wrappers.
+    // Unregister all custom stream wrappers of the parent site.
+    // Availability of Drupal stream wrappers varies by test base class:
+    // - UnitTestBase operates in a completely empty environment.
+    // - DrupalUnitTestBase supports and maintains stream wrappers in a custom
+    //   way.
+    // - WebTestBase re-initializes Drupal stream wrappers are installation.
+    // The original stream wrappers are restored after the test run.
+    // @see TestBase::tearDown()
     $wrappers = file_get_stream_wrappers();
     foreach ($wrappers as $scheme => $info) {
       stream_wrapper_unregister($scheme);
@@ -1112,6 +1119,9 @@ protected function tearDown() {
       }
     }
 
+    // Delete temporary files directory.
+    file_unmanaged_delete_recursive($this->originalFileDirectory . '/simpletest/' . substr($this->databasePrefix, 10), array($this, 'filePreDeleteCallback'));
+
     // Restore original database connection.
     Database::removeConnection('default');
     Database::renameConnection('simpletest_original_default', 'default');
@@ -1139,16 +1149,16 @@ protected function tearDown() {
     \Drupal::setContainer($this->originalContainer);
     $GLOBALS['config_directories'] = $this->originalConfigDirectories;
 
-    // Re-initialize original stream wrappers.
+    // Re-initialize original stream wrappers of the parent site.
+    // This must happen after static variables have been reset and the original
+    // container and $config_directories are restored, as simpletest_log_read()
+    // uses the public stream wrapper to locate the error.log.
     file_get_stream_wrappers();
 
     // In case a fatal error occurred that was not in the test process read the
     // log to pick up any fatal errors.
     simpletest_log_read($this->testId, $this->databasePrefix, get_class($this), TRUE);
 
-    // Delete temporary files directory.
-    file_unmanaged_delete_recursive($this->originalFileDirectory . '/simpletest/' . substr($this->databasePrefix, 10), array($this, 'filePreDeleteCallback'));
-
     if (isset($this->originalPrefix)) {
       drupal_valid_test_ua($this->originalPrefix);
     }