diff -u b/core/includes/bootstrap.inc b/core/includes/bootstrap.inc
--- b/core/includes/bootstrap.inc
+++ b/core/includes/bootstrap.inc
@@ -1134,23 +1134,35 @@
 }
 
 /**
- * Unify PHP 5 and 7's assertion handling.
+ * Registers the assertion handle.
+ *
+ * This assertion handle unifies assertion handling for PHP 5 and PHP 7.
  */
 function drupal_register_assert_handle() {
-  // PHP 7 - just make sure assert failures are thrown.
+  if (!assert_options(ASSERT_ACTIVE)) {
+    return;
+  }
+
+  // In PHP 7, just make sure assert failures are thrown.
   if (version_compare(PHP_VERSION, '7.0.0-dev') >= 0) {
     assert_options(ASSERT_EXCEPTION, 1);
   }
-  // PHP 5 - we must establish an AssertionError to throw and a assert callback
-  // handle to throw it.
+  // In PHP 5, and establish an assert callback handle that will throw AssertionErrors.
   elseif (assert_options(ASSERT_CALLBACK) === NULL) {
-    class AssertionError extends Exception {}
+    class AssertionError extends Exception {
+      public function __construct($message, $code = 0, Exception $previous = NULL, $file, $line) {
+        parent::__construct($message, $code, $previous);
+        // Preserve the filename and line number of the assertion failure.
+        $this->file = $file;
+        $this->line = $line;
+      }
+    }
 
     assert_options(ASSERT_CALLBACK, function($file, $line, $code, $message = '') {
       if (empty($message)) {
-        $message = "Assertion Failure in {$file} at {$line}. Failed asserting {$code}";
+        $message = $code;
       }
-      throw new AssertionError($message);
+      throw new AssertionError($message, 0, NULL, $file, $line);
     });
   }
 }
diff -u b/core/lib/Drupal/Core/DrupalKernel.php b/core/lib/Drupal/Core/DrupalKernel.php
--- b/core/lib/Drupal/Core/DrupalKernel.php
+++ b/core/lib/Drupal/Core/DrupalKernel.php
@@ -862,6 +862,10 @@
         // Simpletest's internal browser.
         define('DRUPAL_TEST_IN_CHILD_SITE', TRUE);
 
+        // Enable runtime assertions.
+        assert_options(ASSERT_ACTIVE, 1);
+        drupal_register_assert_handle();
+
         // Log fatal errors to the test site directory.
         ini_set('log_errors', 1);
         ini_set('error_log', DRUPAL_ROOT . '/sites/simpletest/' . substr($test_prefix, 10) . '/error.log');
@@ -870,10 +874,6 @@
         // Simpletest's internal browser.
         define('DRUPAL_TEST_IN_CHILD_SITE', TRUE);
 
-        // Enable runtime assertions.
-        assert_options(ASSERT_ACTIVE, 1);
-        drupal_register_assert_handle();
-
         // Log fatal errors to the test site directory.
         ini_set('log_errors', 1);
         ini_set('error_log', DRUPAL_ROOT . '/sites/simpletest/' . substr($test_prefix, 10) . '/error.log');
diff -u b/core/modules/simpletest/src/Tests/SimpleTestTest.php b/core/modules/simpletest/src/Tests/SimpleTestTest.php
--- b/core/modules/simpletest/src/Tests/SimpleTestTest.php
+++ b/core/modules/simpletest/src/Tests/SimpleTestTest.php
@@ -168,9 +168,17 @@
     // will be the first of sixteen passes asserted in confirmStubResults()
     try {
       assert(FALSE);
-      $this->fail('Runtime Assertions are not working.');
+      $this->fail('Runtime assertions are not working.');
     } catch (\AssertionError $e) {
-      $this->pass('Runtime Assertions Enabled and running.');
+      $this->pass('Runtime assertions enabled and running.');
+      $this->assertEqual($e->getMessage, 'FALSE');
+    }
+
+    // Check to see that assertion messsages get passed along.
+    try {
+      assert(FALSE, 'Success!');
+    } catch (\AssertionError $e) {
+      $this->assertEqual($e->getMessage, 'Success!');
     }
 
     // This causes the second of the sixteen passes asserted in
@@ -196,7 +204,7 @@
     // confirmStubResults().
     $this->drupalLogin($user);
 
-    // This causes the twelth of the sixteen passes asserted in
+    // This causes the twelfth of the sixteen passes asserted in
     // confirmStubResults().
     $this->pass(t('Test ID is @id.', array('@id' => $this->testId)));
 
diff -u b/core/tests/bootstrap.php b/core/tests/bootstrap.php
--- b/core/tests/bootstrap.php
+++ b/core/tests/bootstrap.php
@@ -93,16 +93,3 @@
-// PHP 7 - just make sure assert failures are thrown.
-if (version_compare(PHP_VERSION, '7.0.0-dev') >= 0) {
-  assert_options(ASSERT_EXCEPTION, 1);
-}
-// PHP 5 - we must establish an AssertionError to throw and a assert callback
-// handle to throw it.
-elseif (assert_options(ASSERT_CALLBACK) === NULL) {
-  class AssertionError extends Exception {}
-
-  assert_options(ASSERT_CALLBACK, function($file, $line, $code, $message = '') {
-    if (empty($message)) {
-      $message = "Assertion Failure in {$file} at {$line}. Failed asserting {$code}";
-    }
-    throw new AssertionError($message);
-  });
-}
+// Register the assertion handle.
+require_once __DIR__ . '/../includes/bootstrap.inc';
+drupal_register_assert_handle();
reverted:
--- b/sites/example.settings.local.php
+++ a/sites/example.settings.local.php
@@ -28,7 +28,6 @@
  * @see https://wiki.php.net/rfc/expectations
  */
 assert_options(ASSERT_ACTIVE, 1);
-drupal_register_assert_handle();
 
 /**
  * Enable local development services.