function composeEventHandlers(theirHandler, ourHandler) {
  return function (event) {
    theirHandler && theirHandler(event);

    if (!event.defaultPrevented) {
      return ourHandler(event);
    }
  };
}
Wraps a lib-defined event handler and a user-defined event handler, returning
a single handler that allows a user to prevent lib-defined handlers from
firing.

@param theirHandler User-supplied event handler
@param ourHandler Library-supplied event handler