function countChildren(children) {
    var n = 0;
    mapChildren(children, function () {
      n++; // Don't return anything
    });
    return n;
  }
Count the number of children that are typically specified as
`props.children`.

See https://reactjs.org/docs/react-api.html#reactchildrencount

@param {?*} children Children tree container.
@return {number} The number of children.