code: function onlyChild(children) {
    ("production" !== "development" ? invariant(
      ReactElement.isValidElement(children),
      'onlyChild must be passed a children with exactly one child.'
    ) : invariant(ReactElement.isValidElement(children)));
    return children;
  }
docs: Returns the first child in a collection of children and verifies that there
  is only one child in the collection. 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 {ReactComponent} The first and only `ReactComponent` contained in the
  structure.