getAndRemove(key) {
    var pair = this.get(key);
    if (!pair) {
      throw new Error('No callbacks for a given key.');
    }
    this.remove(key);
    return pair;
  }
Returns a callback pair, removing it from the map. Throws an error if a
key has no callbacks available.
@param  {string} key
@return {!e2e.ext.utils.CallbackPair}