Down-level lexical 'arguments' binding for arrow functions

[rbuckton]

• Related compatibility test: arrow functions - lexical "arguments" binding

Currently we make this an error when compiling to ES5, however we recently added lexical argument capturing for ES6 async functions in #3078, so we may want to revisit this.

[RyanCavanaugh]

We decided to not bother with this unless someone shows up and asks for it.