The actual source input which caused the error.
This is usually a String, but can be other types too. If it is a string, parts of it may be included in the toString message.
The source is null
if omitted or unknown.
final source
© 2012 the Dart project authors
Licensed under the Creative Commons Attribution-ShareAlike License v4.0.
https://api.dartlang.org/stable/2.0.0/dart-core/FormatException/source.html