The deleted sentinel isn't of type K.

corelib/src/implementation/hash_map_set.dart

Index: corelib/src/implementation/hash_map_set.dart
diff --git a/corelib/src/implementation/hash_map_set.dart b/corelib/src/implementation/hash_map_set.dart
index a05f9f5a4a6374b73189b5411dbef045f7f8e3a1..6290b8f2aa80b05fe07bdb9bddf59a80efbb1fc7 100644
--- a/corelib/src/implementation/hash_map_set.dart
+++ b/corelib/src/implementation/hash_map_set.dart
@@ -228,7 +228,7 @@ class HashMapImplementation<K extends Hashable, V> implements HashMap<K, V> {
   void forEach(void f(K key, V value)) {
     int length = _keys.length;
     for (int i = 0; i < length; i++) {
-      K key = _keys[i];
+      var key = _keys[i];
       if ((key !== null) && (key !== _DELETED_KEY)) {
         f(key, _values[i]);
       }
@@ -261,7 +261,7 @@ class HashMapImplementation<K extends Hashable, V> implements HashMap<K, V> {
   bool containsValue(V value) {
     int length = _values.length;
     for (int i = 0; i < length; i++) {
-      K key = _keys[i];
+      var key = _keys[i];
       if ((key !== null) && (key !== _DELETED_KEY)) {
         if (_values[i] == value) return true;
       }