static register(polyfill, priority) {
    const newItem = {priority: priority || 0, callback: polyfill};
    for (let i = 0; i < shaka.polyfill.polyfills_.length; i++) {
      const item = shaka.polyfill.polyfills_[i];
      if (item.priority < newItem.priority) {
        shaka.polyfill.polyfills_.splice(i, 0, newItem);
        return;
      }
    }
    shaka.polyfill.polyfills_.push(newItem);
  }
Registers a new polyfill to be installed.

@param {function()} polyfill
@param {number=} priority An optional number priority.  Higher priorities
  will be executed before lower priority ones.  Default is 0.
@export