code: function normalizeAttributeKey(key) {
    const hyphenIndex = key.indexOf('-');
  
    if (hyphenIndex !== -1 && key.match(HTML_CUSTOM_ATTR_R) === null) {
      key = key.replace(CAPTURE_LETTER_AFTER_HYPHEN, function(_, letter) {
        return letter.toUpperCase();
      });
    }
  
    return key;
  }
docs: TODO: remove for react 16