groupBy<T, K> method
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<K, List<T>> groupBy<T, K>(K key(T e)) {
var map = <K, List<T>>{};
for (final element in this) {
var list = map.putIfAbsent(key(element as T), () => []);
list.add(element);
}
return map;
}