deliteful/Toaster widget

OK... well, the weird thing here is that you have the check `this.dismissible !== null`, presumably related to the comment "if dismissible was not set", but typically we would just say `dismissible: false` or `dismissible: true` in the prototype, to give it a default value.   In that case `dismissible` is always "set".

Is there some reason you need to do the fancy `!== null` check?