if (TREE_IMPL == null) {
TREE_IMPL = GWT.create(Impl.class);
}
this.style = resources.cellTreeStyle();
this.style.ensureInjected();
initWidget(new SimplePanel());
setStyleName(this.style.cellTreeWidget());
// Initialize the open and close images strings.
ImageResource treeOpen = resources.cellTreeOpenItem();
ImageResource treeClosed = resources.cellTreeClosedItem();