const with for-of not actually ok in all version of node 4

build.js

@@ -22,7 +22,7 @@ const symlinks = {
     'test/img': '../dist/img'
 };
 
-for (const target of Object.keys(symlinks)) {
+for (var target of Object.keys(symlinks)) {
     if (!shell.test('-L', target)) {
         console.log(`Creating symlink:  ${target} -> ${symlinks[target]}`);
         shell.ln('-sf', symlinks[target], target);