Add Iterable.occurrences helper.

lib/src/collection/iterable/count.dart

@@ -7,4 +7,14 @@ extension CountIterableExtension<E> on Iterable<E> {
   /// [1, 2, 3].count((element) => element.isOdd);
   /// ```
   int count(bool Function(E element) predicate) => where(predicate).length;
+
+  /// Returns the number of times [element] appears in the iterable.
+  ///
+  /// The following expression yields 2, because the number 5 appears
+  /// twice:
+  ///
+  /// ```dart
+  /// [1, 5, 4, 5, 2].occurrences(5);
+  /// ```
+  int occurrences(E element) => count((each) => each == element);
 }

lib/src/collection/sortedlist.dart

@@ -42,7 +42,7 @@ class SortedList<E> extends ListBase<E> implements PriorityQueue<E> {
   bool contains(Object? element) =>
       element is E && _comparator.binarySearch(_values, element) >= 0;
 
-  /// Returns the number of times the element appears in the sorted list.
+  /// Returns the number of times [element] appears in the list.
   int occurrences(E element) {
     final lower = _comparator.binarySearchLower(_values, element);
     final upper = _comparator.binarySearchUpper(_values, element);

test/collection_test.dart

@@ -933,6 +933,12 @@ void main() {
         expect(<int>[1, 2, 3].count((each) => each.isOdd), 2);
         expect(<int>[1, 2, 3, 4, 5].count((each) => each.isOdd), 3);
       });
+      test('occurrences', () {
+        expect(<int>[].occurrences(5), 0);
+        expect(<int>[5].occurrences(5), 1);
+        expect(<int>[1].occurrences(5), 0);
+        expect(<int>[1, 5, 4, 5, 2].occurrences(5), 2);
+      });
     });
     group('flatMap', () {
       test('empty', () {