src/vs/editor/contrib/folding/browser/folding.ts/FoldingAction/getLineNumbers
class FoldingAction:	protected getLineNumbers(args: FoldingArguments, editor: ICodeEditor) {
		if (args && args.selectionLines) {
			return args.selectionLines.map(l => l + 1); // to 0-bases line numbers
		}
		return this.getSelectedLines(editor);
	}