function findDeepestCachedAncestor(targetID) {
  deepestNodeSoFar = null;
  ReactInstanceHandles.traverseAncestors(targetID, findDeepestCachedAncestorImpl);

  var foundNode = deepestNodeSoFar;
  deepestNodeSoFar = null;
  return foundNode;
}
Return the deepest cached node whose ID is a prefix of `targetID`.