wasParsed method
Returns true if the option with name was parsed from an actual
argument.
Returns false if it wasn't provided and the default value or no default
value would be used instead.
name must be a valid option name in the parser.
Implementation
bool wasParsed(String name) {
if (!_parser.options.containsKey(name)) {
throw ArgumentError('Could not find an option named "--$name".');
}
return _parsed.containsKey(name);
}