function forEachChildren(children, forEachFunc, forEachContext) {
  if (children == null) {
    return children;
  }
  var traverseContext = getPooledTraverseContext(null, null, forEachFunc, forEachContext);
  traverseAllChildren(children, forEachSingleChild, traverseContext);
  releaseTraverseContext(traverseContext);
}
Iterates through children that are typically specified as `props.children`.

See https://facebook.github.io/react/docs/react-api.html#react.children.foreach

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.