Parse error when using "async" as a parameter name.

[danez]

Reported by @juandopazo

When using async as a parameter name, we get an "Unexpected token" error.

See http://babeljs.io/repl/#?evaluate=false&lineWrap=false&presets=es2015%2Creact%2Cstage-2&experimental=false&loose=false&spec=false&code=const%20test%20%3D%20(%7Basync%20%3D%20true%7D)%20%3D%3E%20%7B%0A%E2%80%82%E2%80%82console.log(async)%3B%0A%7D%3B

const test = ({async = true}) => {
  console.log(async);
};