Fix typo in unmodifiable map mixin return type.

Fixes #44264.

Bug: http://dartbug.com/44264
Change-Id: I958403af0e217cea6b45b4a74cb00fb800914f86
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/173724
Reviewed-by: Kevin Moore <kevmoo@google.com>
Commit-Queue: Lasse R.H. Nielsen <lrn@google.com>
diff --git a/sdk/lib/collection/maps.dart b/sdk/lib/collection/maps.dart
index 90ecb6a..b0ab270 100644
--- a/sdk/lib/collection/maps.dart
+++ b/sdk/lib/collection/maps.dart
@@ -285,7 +285,7 @@
   }
 
   /// This operation is not supported by an unmodifiable map.
-  V remove(Object? key) {
+  V? remove(Object? key) {
     throw UnsupportedError("Cannot modify unmodifiable map");
   }