function traverseAllChildren(children, callback, traverseContext) {
  if (children !== null && children !== undefined) {
    traverseAllChildrenImpl(children, '', 0, callback, traverseContext);
  }
}
Traverses children that are typically specified as `props.children`, but
might also be specified through attributes:

- `traverseAllChildren(this.props.children, ...)`
- `traverseAllChildren(this.props.leftPanelChildren, ...)`

The `traverseContext` is an optional argument that is passed through the
entire traversal. It can be used to store accumulations or anything else that
the callback might find relevant.

@param {?*} children Children tree object.
@param {!function} callback To invoke upon traversing each child.
@param {?*} traverseContext Context for traversal.