parse method
Parses source as a, possibly signed, integer literal and returns its
value.
The source must be a non-empty sequence of base-radix digits,
optionally prefixed with a minus or plus sign ('-' or '+').
The radix must be in the range 2..36. The digits used are
first the decimal digits 0..9, and then the letters 'a'..'z' with
values 10 through 35. Also accepts upper-case letters with the same
values as the lower-case ones.
If no radix is given then it defaults to 10. In this case, the source
digits may also start with 0x, in which case the number is interpreted
as a hexadecimal literal, which effectively means that the 0x is ignored
and the radix is instead set to 16.
For any int n and radix r, it is guaranteed that
n == int.parse(n.toRadixString(r), radix: r).
Throws a FormatException if the source is not a valid integer literal,
optionally prefixed by a sign.
Implementation
external static BigInt parse(String source, {int radix});