Add an extension lastBy on Iterable

lib/src/iterable_extensions.dart

@@ -7,6 +7,7 @@ import 'dart:math' show Random;
 import 'package:collection/src/utils.dart';
 
 import 'algorithms.dart';
+import 'functions.dart' as functions;
 
 /// Extensions that apply to all iterables.
 ///
@@ -353,6 +354,12 @@ extension IterableExtension<T> on Iterable<T> {
     return null;
   }
 
+  /// Associates the elements in [this] by the value returned by [key].
+  ///
+  /// Returns a map from keys computed by [key] to the last value for which [key]
+  /// returns that key.
+  Map<K, T> lastBy<K>(K Function(T) key) => functions.lastBy(this, key);
+
   /// Groups elements by [keyOf] then folds the elements in each group.
   ///
   /// A key is found for each element using [keyOf].

test/extensions_test.dart

@@ -520,6 +520,25 @@ void main() {
           expect(iterable([1, 3, 5]).singleOrNull, null);
         });
       });
+      group('.lastBy', () {
+        test('empty', () {
+          expect(iterable([]).lastBy((dynamic _) {}), {});
+        });
+        test('single', () {
+          expect(iterable([1]).lastBy(toString), {
+            '1': 1,
+          });
+        });
+        test('multiple', () {
+          expect(
+            iterable([1, 2, 3, 4, 5]).lastBy((x) => x.isEven),
+            {
+              false: 5,
+              true: 4,
+            },
+          );
+        });
+      });
       group('.groupFoldBy', () {
         test('empty', () {
           expect(iterable([]).groupFoldBy(unreachable, unreachable), {});