JavaScript breaks on IE <= 8

The M.util.js_complete tests introduced by MDL-41030 use Array.indexOf.
Unfortunately IE8 and below do not support indexOf.

Although we don't support IE8, we shouldn't break this badly.

Possible solutions include:

1. return early if indexOf does not exit on the Array;
2. switch to YUI's Array functions; and
3. monkey patch indexOf onto the Array.

Since we don't support IE8, nor do we test it, I think we should just exit if the indexOf function doesn't exist.