traverse method
      
void
traverse(
    
- dynamic callback(- Object3D
 
inherited
    callback - A function with as first argument an object3D object.
Executes the callback on this object and all descendants.
Note: Modifying the scene graph inside the callback is discouraged.
Implementation
void traverse(Function(Object3D) callback) {
  callback(this);
  final children = this.children;
  for (int i = 0, l = children.length; i < l; i++) {
    children[i].traverse(callback);
  }
}