Switch to non-deprecated fs.existsSync.

Former-commit-id: 616863cf7e7edfdf919750773a12e8cd2a42ddf1

build/minify.js

@@ -54,7 +54,7 @@
    */
   function Minify(source, workingName, onComplete) {
     // create the destination directory if it doesn't exist
-    if (!path.existsSync(distPath)) {
+    if (!fs.existsSync(distPath)) {
       fs.mkdirSync(distPath);
     }