addChild method
Adds a child to this node.
The same node cannot be added to multiple nodes.
addChild(Sprite(myImage));
Implementation
void addChild(Node child) {
assert(child._parent == null);
assert(_assertNonCircularAssignment(child));
_childrenNeedSorting = true;
_children.add(child);
child._parent = this;
child._spriteBox = _spriteBox;
_childrenLastAddedOrder += 1;
child._addedOrder = _childrenLastAddedOrder;
if (_spriteBox != null) _spriteBox!._registerNode(child);
}