Running external code without prototype extensions

I am encountering bugs and performance hits when attempting to refactor an app to avoid using prototype extensions. Is there a way to disable (or enable) prototype extensions in a selective way?

The reason this is necessary is that ShareThis throws an error because a for…in loop on an empty array is entered with extended properties (i.e. “firstObject”). The error might have been tolerable except that IE9 halts the loading of the app at this point.

What is “ShareThis”?