Throw when we try to get or set an unrecognized field

[jtrussell]

class Klassy {
  static get fields() {
    return ['foo', 'bar']
  }
}

const m = new Klassy()
m.get('blargus') // just returns undefined :(
m.get('fooo') // also just returns undefined, silly typo

let m2 = new Klassy()
m2 = m2.set('blargus', 5) // just does it!
m2.get('blargus') // returns 5 :(