fix firefox hoisting issue by defining mixinProp() function outside of for loop(block)

In reference to the following issue,
#43

"The way the function mixinProp is currently being defined in the provider ARMixin doesn't work with firefox.

Firefox isn't hoisting mixinProp() as its being defined within a for loop and it is causing the following error:

Error: mixinProp is not defined

Please see gist and plunkr(open in firefox vs. open in chrome) to see the differences in hoisting

https://gist.github.com/jamesjtong/10272048
http://plnkr.co/edit/qrEuUUzDcwWjQFYILsaC?p=preview"