src/vs/workbench/api/common/extHostTreeViews.ts/ExtHostTreeViews/convertArgument
class ExtHostTreeViews:	private convertArgument(arg: TreeViewItemHandleArg | TreeViewPaneHandleArg): any {
		const treeView = this.treeViews.get(arg.$treeViewId);
		if (treeView && '$treeItemHandle' in arg) {
			return treeView.getExtensionElement(arg.$treeItemHandle);
		}
		if (treeView && '$focusedTreeItem' in arg && arg.$focusedTreeItem) {
			return treeView.focusedElement;
		}
		return null;
	}