Unbreak the build by changing back the type

This commit is supposed to succeed passed the
before_script stage on Travis

lib/utils/async.html

@@ -64,7 +64,7 @@
        * delay.
        *
        * @memberof Polymer.Async.timeOut
-       * @param {number} delay Time to wait before calling callbacks in ms
+       * @param {number=} delay Time to wait before calling callbacks in ms
        * @return {!AsyncInterface} An async timeout interface
        */
       after(delay) {