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;
	}
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.