src/vs/workbench/api/common/extHostTreeViews.ts/ExtHostTreeView/getTooltip
class ExtHostTreeView:	private getTooltip(tooltip?: string | vscode.MarkdownString): string | IMarkdownString | undefined {
		if (MarkdownStringType.isMarkdownString(tooltip)) {
			return MarkdownString.from(tooltip);
		}
		return tooltip;
	}