src/vs/workbench/api/common/extHostTreeViews.ts/ExtHostTreeView/getCheckbox
class ExtHostTreeView:	private getCheckbox(extensionTreeItem: vscode.TreeItem2): ITreeItemCheckboxState | undefined {
		if (!extensionTreeItem.checkboxState) {
			return undefined;
		}
		let checkboxState: TreeItemCheckboxState;
		let tooltip: string | undefined = undefined;
		if (typeof extensionTreeItem.checkboxState === 'number') {
			checkboxState = extensionTreeItem.checkboxState;
		}
		else {
			checkboxState = extensionTreeItem.checkboxState.state;
			tooltip = extensionTreeItem.checkboxState.tooltip;
		}
		return { isChecked: checkboxState === TreeItemCheckboxState.Checked, tooltip };
	}