fixed missing constant issue

lib/font-face/font-face-proxy.js

@@ -38,6 +38,7 @@ this.fontFaceProxy = this.fontFaceProxy || (() => {
 	};
 
 	function hook() {
+		const NEW_FONT_FACE_EVENT = "single-file-new-font-face";
 		const FONT_STYLE_PROPERTIES = {
 			family: "font-family",
 			style: "font-style",