code: function invokeCallback(id) {
  	this[IS_PENDING][id] = true;
  	this[CALLBACKS][id](this[PENDING_ACTION]);
  	this[EMIT_CHANGE_CALLBACK].get(id)();
  	this[IS_HANDLED][id] = true;
  }
docs: Call the callback stored with the given id. Also do some internal
  bookkeeping.
  
  @param {string} id
  @internal