add method
Adds element
to the receiver occurrences
number of times.
Throws an ArgumentError if occurrences
is negative.
Implementation
void add(E element, [int occurrences = 1]) {
if (occurrences < 0) {
throw ArgumentError.value(
occurrences, 'occurrences', 'Negative number of occurrences');
} else if (occurrences > 0) {
_container[element] = this[element] + occurrences;
_length += occurrences;
}
}