Crash in Safari 11.1.2

[warpdesign]

When using the polyfill in Sierra (10.12.6) and Safari 11.1.2, it crashes on the following code:

  Object.defineProperty(AudioContext.prototype, 'audioWorklet', {
    get () {
      return this.$$audioWorklet || (this.$$audioWorklet = new window.AudioWorklet(this));
    }
  });

AudioContext isn't defined so this gives a Can't find variable AudioContext.