function getSelectionOffsetKeyForNode(node: Node): ?string {
  if (isElement(node)) {
    const castedNode: Element = (node: any);
    const offsetKey = castedNode.getAttribute('data-offset-key');
    if (offsetKey) {
      return offsetKey;
    }
    for (let ii = 0; ii < castedNode.childNodes.length; ii++) {
      const childOffsetKey = getSelectionOffsetKeyForNode(
        castedNode.childNodes[ii],
      );
      if (childOffsetKey) {
        return childOffsetKey;
      }
    }
  }
  return null;
}
Get offset key from a node or it's child nodes. Return the first offset key
found on the DOM tree of given node.