function onlyChild(children) {
  !ReactElement.isValidElement(children) ? "development" !== 'production' ? invariant(false, 'onlyChild must be passed a children with exactly one child.') : invariant(false) : undefined;
  return children;
}
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.