CustomEvent.detail is always null in Dartium

[DartBot]

This issue was originally filed by sammcca...@google.com

---

Javascript:
  var evt = new CustomEvent('Event');
  evt.initCustomEvent('Custom', true, true, 'hello from js');
  someElement.dispatchEvent(evt);

Dart:
  someElement.$dom_addEventListener('Custom', (e) {
    print('dart got message: ' + e.detail);
  });

Prints "dart got message: null".
A similar listener in JS can see the detail message fine.

Dartium 21.0.1154.0 (139216)

[madsager]

Added Area-Dartium, Triaged labels.

[DartBot]

This comment was originally written by antonm@google.com

---

Set owner to antonm@google.com.
Added this to the M1 milestone.
Added Started label.

[DartBot]

This comment was originally written by antonm@google.com

---

Added Fixed label.