Index: src/wp-admin/includes/update-core.php
===================================================================
--- src/wp-admin/includes/update-core.php	(revision 45198)
+++ src/wp-admin/includes/update-core.php	(working copy)
@@ -785,6 +785,9 @@
 	// 5.1
 	'wp-includes/random_compat/random_bytes_openssl.php',
 	'wp-includes/js/tinymce/wp-tinymce.js.gz',
+	// 5.3
+	'wp-includes/js/wp-a11y.js', // Moved to: wp-includes/js/dist/a11y.js
+	'wp-includes/js/wp-a11y.min.js', // Moved to: wp-includes/js/dist/a11y.min.js
 );
 
 /**
Index: src/wp-includes/script-loader.php
===================================================================
--- src/wp-includes/script-loader.php	(revision 45198)
+++ src/wp-includes/script-loader.php	(working copy)
@@ -879,8 +879,6 @@
 		)
 	);
 
-	$scripts->add( 'wp-a11y', "/wp-includes/js/wp-a11y$suffix.js", array( 'jquery' ), false, 1 );
-
 	$scripts->add( 'sack', "/wp-includes/js/tw-sack$suffix.js", array(), '1.6.1', 1 );
 
 	$scripts->add( 'quicktags', "/wp-includes/js/quicktags$suffix.js", array(), false, 1 );
Index: tests/phpunit/tests/dependencies/scripts.php
===================================================================
--- tests/phpunit/tests/dependencies/scripts.php	(revision 45198)
+++ tests/phpunit/tests/dependencies/scripts.php	(working copy)
@@ -687,25 +687,46 @@
 		global $wp_scripts;
 
 		wp_default_scripts( $wp_scripts );
+		wp_default_packages( $wp_scripts );
 
 		$wp_scripts->base_url  = '';
 		$wp_scripts->do_concat = true;
 
 		$ver       = get_bloginfo( 'version' );
-		$expected  = "<script type='text/javascript' src='/wp-admin/load-scripts.php?c=0&amp;load%5B%5D=jquery-core,jquery-migrate,wp-a11y&amp;ver={$ver}'></script>\n";
+
+		$expected = "<script type='text/javascript' src='/wp-admin/load-scripts.php?c=0&amp;load%5B%5D=jquery-core,jquery-migrate&amp;ver={$ver}'></script>\n";
 		$expected .= "<script type='text/javascript'>\nconsole.log(\"before\");\n</script>\n";
-		$expected .= "<script type='text/javascript' src='http://example.com'></script>\n";
-		$expected .= "<script type='text/javascript' src='http://example2.com'></script>\n";
+		$expected .= "<script type='text/javascript' src='http://example.org'></script>\n";
+		$expected .= "<script type='text/javascript' src='/wp-includes/js/dist/vendor/wp-polyfill.min.js'></script>\n";
+		$expected .= "<script type='text/javascript'>\n";
+		$expected .= "( 'fetch' in window ) || document.write( '<script src=\"http://example.org/wp-includes/js/dist/vendor/wp-polyfill-fetch.min.js\"></scr' + 'ipt>' );( document.contains ) || document.write( '<script src=\"http://example.org/wp-includes/js/dist/vendor/wp-polyfill-node-contains.min.js\"></scr' + 'ipt>' );( window.FormData && window.FormData.prototype.keys ) || document.write( '<script src=\"http://example.org/wp-includes/js/dist/vendor/wp-polyfill-formdata.min.js\"></scr' + 'ipt>' );( Element.prototype.matches && Element.prototype.closest ) || document.write( '<script src=\"http://example.org/wp-includes/js/dist/vendor/wp-polyfill-element-closest.min.js\"></scr' + 'ipt>' );\n";
+		$expected .= "</script>\n";
+		$expected .= "<script type='text/javascript' src='/wp-includes/js/dist/dom-ready.min.js'></script>\n";
+		$expected .= "<script type='text/javascript' src='/wp-includes/js/dist/a11y.min.js'></script>\n";
+		$expected .= "<script type='text/javascript' src='http://example2.org'></script>\n";
 		$expected .= "<script type='text/javascript'>\nconsole.log(\"after\");\n</script>\n";
 
-		wp_enqueue_script( 'test-example', 'http://example.com', array( 'jquery' ), null );
+		wp_enqueue_script( 'test-example', 'http://example.org', array( 'jquery' ), null );
 		wp_add_inline_script( 'test-example', 'console.log("before");', 'before' );
-		wp_enqueue_script( 'test-example2', 'http://example2.com', array( 'wp-a11y' ), null );
+		wp_enqueue_script( 'test-example2', 'http://example2.org', array( 'wp-a11y' ), null );
 		wp_add_inline_script( 'test-example2', 'console.log("after");', 'after' );
 
+		// Open a buffer to get the output of `wp_print_scripts`.
+		ob_start();
 		wp_print_scripts();
-		$print_scripts = get_echo( '_print_scripts' );
+		$print_scripts = ob_get_clean();
 
+		// We've replaced wp-a11y.js with @wordpress/a11y package (see 45066)
+		// and `wp-polyfill` is now a dependency of the packaged wp-a11y.
+		// The packaged scipts contain various version numbers, which are
+		// not exposed, so we will remove all version args from the output.
+		$print_scripts = preg_replace(
+			'~js\?ver=([^"\']*)~', // Matches `js?ver=X.X.X` and everything to single or double quote.
+			'js', // The replacement, `js` without the version arg.
+			$print_scripts // Printed scripts.
+		);
+
+		// Check if expected and printed scripts are equal.
 		$this->assertEquals( $expected, $print_scripts );
 	}
 
@@ -716,12 +737,12 @@
 		global $wp_scripts;
 
 		wp_default_scripts( $wp_scripts );
+		wp_default_packages( $wp_scripts );
 
 		$wp_scripts->base_url  = '';
 		$wp_scripts->do_concat = true;
 
-		$expected_tail  = "<![endif]-->\n";
-		$expected_tail .= "<script type='text/javascript' src='/customize-dependency.js'></script>\n";
+		$expected_tail = "<script type='text/javascript' src='/customize-dependency.js'></script>\n";
 		$expected_tail .= "<script type='text/javascript'>\n";
 		$expected_tail .= "tryCustomizeDependency()\n";
 		$expected_tail .= "</script>\n";
@@ -730,10 +751,12 @@
 		wp_enqueue_script( $handle, '/customize-dependency.js', array( 'customize-controls' ), null );
 		wp_add_inline_script( $handle, 'tryCustomizeDependency()' );
 
+		// Open a buffer to get the output of `wp_print_scripts`.
+		ob_start();
 		wp_print_scripts();
-		$print_scripts = get_echo( '_print_scripts' );
+		$print_scripts = ob_get_clean();
 
-		$tail = substr( $print_scripts, strrpos( $print_scripts, '<![endif]-->' ) );
+		$tail = substr( $print_scripts, strrpos( $print_scripts, "<script type='text/javascript' src='/customize-dependency.js'>" ) );
 		$this->assertEquals( $expected_tail, $tail );
 	}
 
Index: tests/qunit/index.html
===================================================================
--- tests/qunit/index.html	(revision 45198)
+++ tests/qunit/index.html	(working copy)
@@ -24,7 +24,7 @@
 			};
 		</script>
 		<script src="../../build/wp-includes/js/wp-util.js"></script>
-		<script src="../../build/wp-includes/js/wp-a11y.js"></script>
+		<script src="../../build/wp-includes/js/dist/a11y.js"></script>
 		<script>
 			window._wpMediaModelsL10n = {"settings":{"ajaxurl":"\/wp-admin\/admin-ajax.php","post":{"id":0}}};
 		</script>