You could use `+this.textureCache_.peekLastKey()` to convert the key to a date number. This [article](http://www.js-attitude.fr/2012/12/26/convertir-un-nombre-en-texte-en-javascript/?utm_source=jsattitude&utm_medium=twitter&utm_campaign=numeric-conversion) (french) states than using `+` is equivalent to using `Number`.  FWIW Closure uses `+new Date()` to convert the date to number (in browsers that don't have `Date.now`.