Deprecate index()'s false return value

[nex3]

79bad1f changed index() from returning false when not finding an element to returning null. As discussed in #832, we didn't think this necessitated a deprecation process since we expected users to be writing @if index(...). However, it seems that at least Foundation explicitly checks against false.

Rather than returning null immediately, we'll need to add a DeprecatedFalse value that complains if it's used in a way that is incompatible with the future null class.

[mikehdt]

I've used a bit of "x == false" stuff in some cases where I care only about if something doesn't exist in a list.

Given we can't (or shouldn't) do x == false, and x == null also throws a warning, what should we do? I can only really think of checking for a "positive" value, and then immediately following it with an @else. But that seems kludgy.

[nex3]

Why can't you write @if not x?

[mikehdt]

You can - perfect! Thanks :) The warning from the compiler only suggested using @if x, hence why I was trying to figure a way around it.

Perhaps rather than throwing For example, instead of "@if index(...) == false", just write "@if index(...)". it could suggest @if not index(...) instead if that makes sense? (That message was from the Sass compiler in CodeKit 2).

[nex3]

It's tough to get enough contextual information to figure out the perfect error message, unfortunately :-/.

[nex3]

Actually, that error should just always say @if not index(...). I'll fix that.