addCallbacks(callback, errback) {
    var requestId;
    do {
      requestId = goog.string.getRandomString();
    } while (this.containsKey(requestId));
    this.set(requestId, {callback: callback, errback: errback});
    return requestId;
  }
Inserts a pair of callbacks into the map under a new unique key.
@param {function(...)} callback The callback function.
@param {function(Error)} errback The function that will be called upon
    error.
@return {string} The key under which the callback have been stored.