src/vs/nls.mock.ts/_format
function _format(message: string, args: any[]): string {
	let result: string;
	if (args.length === 0) {
		result = message;
	} else {
		result = message.replace(/\{(\d+)\}/g, function (match, rest) {
			const index = rest[0];
			return typeof args[index] !== 'undefined' ? args[index] : match;
		});
	}
	return result;
}