countOccurrences method
Counts non-overlapping occurrences of pattern.
Implementation
int countOccurrences(String pattern) {
if (pattern.isEmpty) return 0;
var count = 0;
var start = 0;
while (true) {
final index = indexOf(pattern, start);
if (index == -1) break;
count++;
start = index + pattern.length;
}
return count;
}