function checkSymbolNodeType(node: ?AnyParseNode): ?SymbolParseNode {
    if (node && (node.type === "atom" || NON_ATOMS.hasOwnProperty(node.type))) {
        // $FlowFixMe
        return node;
    }
    return null;
}
Returns the node more strictly typed iff it is of the given type. Otherwise,
returns null.