How to route to a controller action in Craft3?
The problem ended up being something underlying in craft or yii2 that will not route the word "echo".

I changed out "echo" to "test" and it works fine. Even with both routes in, one does not and one does:

    // Register our site routes
    Event::on(
        UrlManager::class,
        UrlManager::EVENT_REGISTER_SITE_URL_RULES,
        function (RegisterUrlRulesEvent $event) {
            $event-&gt;rules['echo'] = 'site-dom-injector/default/index';
            $event-&gt;rules['test'] = 'site-dom-injector/default/index';
        }
    );