function ForEachBookKeeping(forEachFunction, forEachContext) {
  this.forEachFunction = forEachFunction;
  this.forEachContext = forEachContext;
}
Maps children that are typically specified as `props.children`.

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

TODO: This may likely break any calls to `ReactChildren.map` that were
previously relying on the fact that we guarded against null children.

@param {?*} children Children tree container.
@param {function(*, int)} mapFunction.
@param {*} mapContext Context for mapFunction.
@return {object} Object containing the ordered map of results.