code: function onlyChild(children) {
    !ReactElement.isValidElement(children) ? process.env.NODE_ENV !== 'production' ? invariant(false, 'React.Children.only expected to receive a single React element child.') : _prodInvariant('143') : void 0;
    return children;
  }
docs: Returns the first child in a collection of children and verifies that there
  is only one child in the collection.
  
  See https://facebook.github.io/react/docs/top-level-api.html#react.children.only
  
  The current implementation of this function assumes that a single child gets
  passed without a wrapper, but the purpose of this helper function is to
  abstract away the particular structure of children.
  
  @param {?object} children Child collection structure.
  @return {ReactElement} The first and only `ReactElement` contained in the
  structure.