Switch from `let` to `var` to support older versions of node.

scripts/watch-filter.js

@@ -1,6 +1,6 @@
 var path = require('path');
 
 module.exports = function(f, stat) {
-    let parts = f.split(path.sep);
+    var parts = f.split(path.sep);
     return parts.indexOf('src') !== -1 || parts.indexOf('style') !== -1;
 }