It looks like you need to add something to the prefix comment for the preprocessor to catch it, changing it to e.g. `/* -ms-a */` should work. Could you please fix this and squash the commits?
I'm sorry about giving you the wrong information earlier, I didn't look closely enough at the [regexp](https://github.com/mozilla/pdf.js/blob/master/external/builder/builder.js#L118)!