diff --git a/core/modules/simpletest/src/Tests/SimpleTestTest.php b/core/modules/simpletest/src/Tests/SimpleTestTest.php
index 559ff53..b78e591 100644
--- a/core/modules/simpletest/src/Tests/SimpleTestTest.php
+++ b/core/modules/simpletest/src/Tests/SimpleTestTest.php
@@ -267,6 +267,19 @@ function confirmStubTestResults() {
 
     $this->assertEqual('16 passes, 3 fails, 2 exceptions, 4 debug messages', $this->childTestResults['summary']);
 
+    // Assert that there exists an assertion of type "Duration" and that the
+    // assertion message contains a numeric value (the duration in seconds).
+    $duration_assert_found = FALSE;
+    $duration_value_is_numeric = FALSE;
+    foreach ($this->childTestResults['assertions'] as $assertion) {
+      if ($assertion['type'] == 'Duration') {
+        $duration_assert_found = TRUE;
+        $duration_value_is_numeric = is_numeric($assertion['message']);
+      }
+    }
+    $this->assertTrue($duration_assert_found, 'Duration assert found.');
+    $this->assertTrue($duration_value_is_numeric, 'Duration value is numeric.');
+
     $this->testIds[] = $test_id = $this->getTestIdFromResults();
     $this->assertTrue($test_id, 'Found test ID in results.');
   }