function countChildren(children, context) {
	  return traverseAllChildren(children, forEachSingleChildDummy, null);
	}
Count the number of children that are typically specified as
`props.children`.

See https://facebook.github.io/react/docs/top-level-api.html#react.children.count

@param {?*} children Children tree container.
@return {number} The number of children.