chore(utils): do not mutate source arrays

Author: petebacondarwin

Diff for: lib/grunt/utils.js

@@ -75,10 +75,8 @@ module.exports = {
   },
 
 
-  wrap: function(src, name) {
-    src.unshift('src/' + name + '.prefix');
-    src.push('src/' + name + '.suffix');
-    return src;
+  wrap(src, name) {
+    return [`src/${name}.prefix`, ...src, `src/${name}.suffix`];
   },