groupBy<K> method
Groups elements of the original collection by the key returned by the
given keySelector
function applied to each element and returns a map.
Each group key is associated with a list of corresponding elements.
The returned map preserves the entry iteration order of the keys produced from the original collection.
Implementation
Map<K, List<E>> groupBy<K>(K Function(E element) keySelector) {
return collection.groupBy(this, keySelector);
}