checkAsyncFunctionStart example

    protected function parseFunctionOrGeneratorDeclaration(
        $default = false, $allowGenerator = true
    ) {
        $async = null;
        if ($this->features->asyncAwait &&
            ($async = $this->checkAsyncFunctionStart())) {
            $this->scanner->consumeToken();
            if (!$this->features->asyncIterationGenerators) {
                $allowGenerator = false;
            }
        }
        if ($token = $this->scanner->consume("function")) {

            $generator = $allowGenerator && $this->scanner->consume("*");
            $id = $this->parseIdentifier(static::$bindingIdentifier);

            if ($generator || $async) {