Add a failing test for handling `arguments` correctly.

Per the [relevant section of the ES6 spec](http://people.mozilla.org/~jorendorff/es6-draft.html#sec-arrow-function-definitions-runtime-semantics-evaluation), `arguments` inside an arrow function refers to the `arguments` in the lexically enclosing function.

I tried to fix this using a `case infix` macro, but I don't understand it well enough to know exactly how to start.