offset property
int offsetfinal
The offset in source where the error was detected.
A zero-based offset into the source that marks the format error causing this exception to be created. If source
is a string, this should be a string index in the range 0 <= offset <= source.length
.
If input is a string, the toString method may represent this offset as a line and character position. The offset should be inside the string, or at the end of the string.
May be omitted. If present, source should also be present if possible.
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https://api.dartlang.org/stable/1.24.3/dart-core/FormatException/offset.html