Index: tests/phpunit/tests/dependencies/mediaelementjs.php
===================================================================
--- tests/phpunit/tests/dependencies/mediaelementjs.php	(nonexistent)
+++ tests/phpunit/tests/dependencies/mediaelementjs.php	(working copy)
@@ -0,0 +1,36 @@
+<?php
+
+/**
+ * @group dependencies
+ * @group scripts
+ */
+class Tests_Dependencies_MediaElementjs extends WP_UnitTestCase {
+	/**
+	 * Test if the MediaElement.js Flash fallbacks have been re-added.
+	 *
+	 * MediaElement's Flash fallbacks were removed in WordPress 4.9.2 due to limited use cases and
+	 * a history of security vulnerabilities. It's unlikely that there'll ever be a need to
+	 * restore them in the future, and doing so could introduce security vulnerabilities. If you
+	 * want to re-add them, please discuss that with the Security team first.
+	 *
+	 * @since 5.0.0
+	 *
+	 * @ticket 42720
+	 */
+	function test_exclusion_of_flash() {
+		$mejs_folder = dirname( ABSPATH ) . '/build/' . WPINC . '/js/mediaelement';
+		$js_files    = glob( $mejs_folder . '/*.js' );
+
+		/*
+		 * The path in $mejs_folder is hardcoded, so this is just a sanity check to make sure the
+		 * correct directory is used, in case it gets renamed in the future.
+		 */
+		$this->assertGreaterThan( 0, count( $js_files ) );
+
+		$mejs_directory_iterator = new RecursiveIteratorIterator( new RecursiveDirectoryIterator( $mejs_folder ) );
+		$mejs_swf_iterator       = new RegexIterator( $mejs_directory_iterator, '/\.swf$/i', RecursiveRegexIterator::GET_MATCH );
+
+		// Make sure the Flash files haven't been re-added accidentally.
+		$this->assertCount( 0, iterator_to_array( $mejs_swf_iterator ) );
+	}
+}