cast is only performed as a check

[johanvos]

This code

    public static void superProcessCSS(Node node) {
        ((ParentHelper) getHelper(node)).superProcessCSSImpl(node);
    }

get transpiled into

    m = c.superProcessCSS__VLjavafx_scene_Node_2 = function(lcA0) {
var stA0 = c.getHelper__Lcom_sun_javafx_scene_NodeHelper_2Ljavafx_scene_Node_2(lcA0);
if (stA0 !== null && !stA0['$instOf_com_sun_javafx_scene_ParentHelper']) vm.java_lang_Class(false).castEx(stA0, 'com.sun.javafx.scene.ParentHelper');
stA0.superProcessCSSImpl__VLjavafx_scene_Node_2(lcA0);
return;
};

The value of stA0 is initially (correctly) assigned to an instance of ControlHelper which extends ParentHelper. When the object is casted to a (ParentHelper), the method invocation should use the method body from the ParentHelper, and not that of the ControlHelper.