groupBy<
Groups the elements in values by the value returned by key.
Returns a map from keys computed by key to a list of all values for which
key returns that key. The values appear in the list in the same relative
order as in values.
Implementation
Map<T, List<S>> groupBy<S, T>(Iterable<S> values, T key(S element)) {
var map = <T, List<S>>{};
for (var element in values) {
var list = map.putIfAbsent(key(element), () => []);
list.add(element);
}
return map;
}