Provide a representation of this double value.
The representation is a number literal such that the closest double value to the representation's mathematical value is this double.
Returns "NaN" for the Not-a-Number value. Returns "Infinity" and "-Infinity" for positive and negative Infinity. Returns "-0.0" for negative zero.
For all doubles,
d, converting to a string and parsing the string back
gives the same value again:
d == double.parse(d.toString()) (except when
d is NaN).