From 5826da67218d552bb920b823e7b3c105b66f1d0b Mon Sep 17 00:00:00 2001
From: William Hesse <whesse@google.com>
Date: Wed, 18 Mar 2020 18:07:30 +0100
Subject: [PATCH] Replace a use of putIfAbsent with ??=
 (dart-lang/collection#125)

Because it avoids using a closure, it seems that ??= [] can be used with better performance than putIfAbsent in groupBy.
---
 pkgs/collection/lib/src/functions.dart | 3 +--
 1 file changed, 1 insertion(+), 2 deletions(-)

diff --git a/pkgs/collection/lib/src/functions.dart b/pkgs/collection/lib/src/functions.dart
index 0f71548a..5fbef305 100644
--- a/pkgs/collection/lib/src/functions.dart
+++ b/pkgs/collection/lib/src/functions.dart
@@ -49,8 +49,7 @@ Map<K, V> mergeMaps<K, V>(Map<K, V> map1, Map<K, V> map2,
 Map<T, List<S>> groupBy<S, T>(Iterable<S> values, T Function(S) key) {
   var map = <T, List<S>>{};
   for (var element in values) {
-    var list = map.putIfAbsent(key(element), () => []);
-    list.add(element);
+    (map[key(element)] ??= []).add(element);
   }
   return map;
 }