The deleted sentinel isn't of type K.

corelib/src/implementation/hash_map_set.dart

 // Copyright (c) 2011, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 // Hash map implementation with open addressing and quadratic probing.
 class HashMapImplementation<K extends Hashable, V> implements HashMap<K, V> {
 
   // The [_keys] list contains the keys inserted in the map.
   // The [_keys] list must be a raw list because it
   // will contain both elements of type K, and the [_DELETED_KEY] of type
@@ skipping 210 matching lines @@
     return _numberOfEntries == 0;
   }
 
   int get length() {
     return _numberOfEntries;
   }
 
   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]);
       }
     }
   }
 
 
   Collection<K> getKeys() {
     List<K> list = new List<K>(length);
     int i = 0;
@@ skipping 12 matching lines @@
     return list;
   }
 
   bool containsKey(K key) {
     return (_probeForLookup(key) != -1);
   }
 
   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;
       }
     }
     return false;
   }
 }
 
 class HashSetImplementation<E extends Hashable> implements HashSet<E> {
 
@@ skipping 157 matching lines @@
 
 /**
  * A singleton sentinel used to represent when a key is deleted from the map.
  * We can't use [: const Object() :] as a sentinel because it would end up
  * canonicalized and then we cannot distinguish the deleted key from the
  * canonicalized [: Object() :].
  */
 class _DeletedKeySentinel {
   const _DeletedKeySentinel();
 }