checkSuffixes method
If the rest of our input is either the positive or negative suffix, set gotPositiveSuffix or gotNegativeSuffix accordingly.
Implementation
void checkSuffixes() {
var remainder = input.peekAll();
if (remainder == _positiveSuffix) gotPositiveSuffix = true;
if (remainder == _negativeSuffix) gotNegativeSuffix = true;
}