What is the experimental feature "indexed members"?

Question

On the new Roslyn Preview site it mentions being able to try out potential language features, and lists three such features.

The first two I've heard of before (for example here), but I can't figure out from the code sample what "indexed members" are. Can anyone explain what these are, either based on another source or the code sample? (It's worth nothing $x is not a valid identifier in C# 5.)

UPDATE - According to the Roslyn Feature Status page, this feature has been withdrawn.

Answer 1

.$foobar is just short form for ["foobar"]