Fix this binding in arrow functions in classes

[SarahIsWeird]

This fixes #236. I'm not sure if there are other weird edge cases this misses, though.

function(c, a) {
  class MyClass extends Object {
    constructor() {
      let _0o2iruz6j10_THIS_ = super()
      _0o2iruz6j10_THIS_.number = 2
    }
    foo() {
      let _0o2iruz6j10_THIS_ = super.valueOf()
      return [1, 2, 3].filter(n => n == _0o2iruz6j10_THIS_.number)
    }
  }
  return new MyClass()
}