src/vs/workbench/api/common/extHostTreeViews.ts/ExtHostTreeView/setCheckboxState
class ExtHostTreeView:	async setCheckboxState(checkboxUpdates: CheckboxUpdate[]) {
		type CheckboxUpdateWithItem = { extensionItem: NonNullable<T>; treeItem: vscode.TreeItem2; newState: TreeItemCheckboxState };
		const items = (await Promise.all(checkboxUpdates.map(async checkboxUpdate => {
			const extensionItem = this.getExtensionElement(checkboxUpdate.treeItemHandle);
			if (extensionItem) {
				return {
					extensionItem: extensionItem,
					treeItem: await this.dataProvider.getTreeItem(extensionItem),
					newState: checkboxUpdate.newState ? TreeItemCheckboxState.Checked : TreeItemCheckboxState.Unchecked
				};
			}
			return Promise.resolve(undefined);
		}))).filter<CheckboxUpdateWithItem>((item): item is CheckboxUpdateWithItem => item !== undefined);

		items.forEach(item => {
			item.treeItem.checkboxState = item.newState ? TreeItemCheckboxState.Checked : TreeItemCheckboxState.Unchecked;
		});

		this._onDidChangeCheckboxState.fire({ items: items.map(item => [item.extensionItem, item.newState]) });
	}