lib: replace String concatenation with template

PR-URL: #16922
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Tobias Nießen <tniessen@tnie.de>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Gireesh Punathil <gpunathi@in.ibm.com>
Reviewed-By: Franziska Hinkelmann <franziska.hinkelmann@gmail.com>

lib/internal/v8_prof_polyfill.js

@@ -79,7 +79,7 @@ var line = '';
 
 function peekline() {
   const s = readline();
-  line = s + '\n' + line;
+  line = `${s}\n${line}`;
   return s;
 }