performance improvement in plugins.js

Performance and speed are something important when it comes to javascript, so I think this little improvement to  the script in `plugins.js` will make sense.

I tested it with JSPerf: http://jsperf.com/testing-this-window-parameter
In every browser I tested, it seems to be a little bit faster.
`this` can refer to something else in certain contexts, plus IMHO it doesn't makes a significant difference. 

(Cc: @mathiasbynens)