toIso8601String method
Returns an ISO-8601 full-precision extended format representation.
The format is yyyy-MM-ddTHH:mm:ss.mmmuuunnnZ
nanoseconds and microseconds are omitted if null
Implementation
String toIso8601String() {
// Use DateTime without the sub second part
var text = Timestamp(seconds, 0).toDateTime(isUtc: true).toIso8601String();
// Then add the nano part to it
var nanosStart = text.lastIndexOf('.') + 1;
return '${text.substring(0, nanosStart)}${_formatNanos(nanoseconds)}Z';
}