lib: replace string concatenation with template

PR-URL: #16933
Reviewed-By: Anatoli Papirovski <apapirovski@mac.com>
Reviewed-By: Franziska Hinkelmann <franziska.hinkelmann@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Gireesh Punathil <gpunathi@in.ibm.com>

lib/fs.js

@@ -115,7 +115,7 @@ function rethrow() {
     var backtrace = new Error();
     return function(err) {
       if (err) {
-        backtrace.stack = err.name + ': ' + err.message +
+        backtrace.stack = `${err.name}: ${err.message}` +
                           backtrace.stack.substr(backtrace.name.length);
         throw backtrace;
       }
@@ -1894,7 +1894,7 @@ fs.mkdtemp = function(prefix, options, callback) {
   var req = new FSReqWrap();
   req.oncomplete = callback;
 
-  binding.mkdtemp(prefix + 'XXXXXX', options.encoding, req);
+  binding.mkdtemp(`${prefix}XXXXXX`, options.encoding, req);
 };
 
 
@@ -1907,7 +1907,7 @@ fs.mkdtempSync = function(prefix, options) {
   }
   options = getOptions(options, {});
   nullCheck(prefix);
-  return binding.mkdtemp(prefix + 'XXXXXX', options.encoding);
+  return binding.mkdtemp(`${prefix}XXXXXX`, options.encoding);
 };