commit 2084dd4545ee96ad0d12f3a9acfb223215dae291
Author: xjm <xjm@65776.no-reply.drupal.org>
Date:   Sat Nov 25 07:50:46 2017 -0600

    98

diff --git a/core/modules/simpletest/src/InstallerTestBase.php b/core/modules/simpletest/src/InstallerTestBase.php
index 151e9eff00..04e94e82b7 100644
--- a/core/modules/simpletest/src/InstallerTestBase.php
+++ b/core/modules/simpletest/src/InstallerTestBase.php
@@ -244,16 +244,26 @@ protected function continueOnExpectedWarnings($allowed_warnings = []) {
     if (strpos($this->getTextContent(), 'Errors found')) {
       return;
     }
-    $warning_elements = $this->cssSelect('h3#warning+details summary');
+    // Allow only details elements that are directly after the warning header
+    // or each other. There is no guaranteed wrapper we can rely on across
+    // distributions. When there are multiple warnings, the selectors will be:
+    // - h3#warning+details summary
+    // - h3#warning+details+details summary
+    // - etc.
+    // We add one more selector than expected warnings to confirm that there
+    // isn't any other warning before clicking the link.
+    $selectors = [];
+    for ($i = 0; $i <= count($allowed_warnings); $i++) {
+      $selectors[] = 'h3#warning' . implode('', array_fill(0, $i + 1, '+details')) . ' summary';
+    }
+    $warning_elements = $this->cssSelect(implode(', ', $selectors));
     // Confirm that there are only the expected warnings.
-    if (count($allowed_warnings) == count($warning_elements)) {
-      $warnings = [];
-      foreach ($warning_elements as $warning) {
-        $warnings[] = trim((string) $warning);
-      }
-      $this->assertEqual($allowed_warnings, $warnings);
-      $this->clickLink('continue anyway');
+    $warnings = [];
+    foreach ($warning_elements as $warning) {
+      $warnings[] = trim((string) $warning);
     }
+    $this->assertEqual($allowed_warnings, $warnings);
+    $this->clickLink('continue anyway');
   }
 
 }