Function once

tests/helpers.js:7–14

(el, eventName, handler)

 * tests and an event emitted in one may be picked up by another, causing test failures.
 */
export function once (el, eventName, handler) {
  var done = false;
  el.addEventListener(eventName, function onceHandler (evt) {
    if (done) { return; }
    handler(evt);
    done = true;
  });
}

/**
 * Helper method to create a scene, create an entity, add entity to scene,