[Bugfix Beta] detect if we can use toString to analyze functions

In some modes platforms like PS3 + FireFox OS will discard these sources to save on memory. Instead we will opt into this perf only after we detect the ability.

cc @krisselden @mixonic @twokul

I tested this in the Firefox OS WebIDE and all seems well. The feature detection appears to work, and our test suite appears 100% green.

Unfortunately I don't have a real firefox OS device, or a PS3 so I can't test further.


Defaulting to `true` here would simplify the conditional below (you could just check `if (hasSuper)` instead of `if (canToStringFunction === false || hasSuper)`).