function forEachChildren(children, forEachFunc, forEachContext) {
    mapChildren(children, function () {
      forEachFunc.apply(this, arguments); // Don't return anything.
    }, forEachContext);
  }
Iterates through children that are typically specified as `props.children`.

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

The provided forEachFunc(child, index) will be called for each
leaf child.

@param {?*} children Children tree container.
@param {function(*, int)} forEachFunc
@param {*} forEachContext Context for forEachContext.