If webcomponents-loader.js is loaded asynchronously and `document.readystate` is still 'loading' `document.write` in [line 161](https://github.com/webcomponents/webcomponentsjs/blob/master/webcomponents-loader.js#L161 ) does not work. In Firefox, for example, it gives the following warning: A call to document.write() from an asynchronously-loaded external script was ignored.
