"export" flag attribute documented in platform.js is actually "exportas"

Minor discrepancy between code and comments: I was trying to get the various polyfill members exposed as a global by following the comments in platform.js, which indicate that you can do

```
<script src="platform.js" export=myglobal></script>
```

However, the actual flag is actually "exportas", not "export". (I wasn't sure whether the code or the comment should be fixed.)
