public void attachLinkedChild(Spatial spat, ModelKey key) {
        addLinkedChild(key);
        assetChildren.put(key, spat);
        attachChild(spat);
    }
Add a "linked" child. These are loaded from the assetManager when the
AssetLinkNode is loaded from a binary file.

@param key the ModelKey to add
jme3-core/src/main/java/com/jme3/scene/AssetLinkNode.java