function getFirstCommonAncestorID(oneID, twoID) {
  var minLength = Math.min(oneID.length, twoID.length);
  if (minLength === 0) {
    return '';
  }
  var lastCommonMarkerIndex = 0;
  // Use `<=` to traverse until the "EOL" of the shorter string.
  for (var i = 0; i <= minLength; i++) {
    if (isBoundary(oneID, i) && isBoundary(twoID, i)) {
      lastCommonMarkerIndex = i;
    } else if (oneID.charAt(i) !== twoID.charAt(i)) {
      break;
    }
  }
  var longestCommonID = oneID.substr(0, lastCommonMarkerIndex);
  !isValidID(longestCommonID) ? "development" !== 'production' ? invariant(false, 'getFirstCommonAncestorID(%s, %s): Expected a valid React DOM ID: %s', oneID, twoID, longestCommonID) : invariant(false) : undefined;
  return longestCommonID;
}
Gets the nearest common ancestor ID of two IDs.

Using this ID scheme, the nearest common ancestor ID is the longest common
prefix of the two IDs that immediately preceded a "marker" in both strings.

@param {string} oneID
@param {string} twoID
@return {string} Nearest common ancestor ID, or the empty string if none.
@private