Reflect.set edge case

[jridgewell]

Currently, the polyfilled Reflect.set fails this case:

const o = Object.defineProperty({}, 'test', {
  writable: false,
  configurable: true
});
Reflect.set(o.__proto__, 'test', 1, o);

This should fail to set, but it currently does.

[zloirock]

Thanks, fixed.