tryParse method Null safety

Parses source as a, possibly signed, integer literal and returns its value.

As parse except that this method returns null if the input is not valid

Examples:

print(BigInt.tryParse('-12345678901234567890')); // -12345678901234567890
print(BigInt.tryParse('0xFF')); // 255
print(BigInt.tryParse('0xffffffffffffffff')); // 18446744073709551615

// From binary (base 2) value.
print(BigInt.tryParse('1100', radix: 2)); // 12
print(BigInt.tryParse('00011111', radix: 2)); // 31
print(BigInt.tryParse('011111100101', radix: 2)); // 2021
// From octal (base 8) value.
print(BigInt.tryParse('14', radix: 8)); // 12
print(BigInt.tryParse('37', radix: 8)); // 31
print(BigInt.tryParse('3745', radix: 8)); // 2021
// From hexadecimal (base 16) value.
print(BigInt.tryParse('c', radix: 16)); // 12
print(BigInt.tryParse('1f', radix: 16)); // 31
print(BigInt.tryParse('7e5', radix: 16)); // 2021
// From base 35 value.
print(BigInt.tryParse('y1', radix: 35)); // 1191 == 34 * 35 + 1
print(BigInt.tryParse('z1', radix: 35)); // null
// From base 36 value.
print(BigInt.tryParse('y1', radix: 36)); // 1225 == 34 * 36 + 1
print(BigInt.tryParse('z1', radix: 36)); // 1261 == 35 * 36 + 1