Ensure that hashCode and runtimeType work on null.

lib/core/map.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.
 
 /**
  * A [Map] is an associative container, mapping a key to a value.
- * Null values are supported.
+ * Null values are supported, but null keys are not.
  */
 interface Map<K, V> default HashMapImplementation<K extends Hashable, V> {
   /**
    * Creates a map with the default implementation.
    */
   Map();
 
   /**
    * Creates a [Map] that contains all key value pairs of [other].
    */
@@ skipping 96 matching lines @@
   /**
    * Creates a map with the default implementation.
    */
   LinkedHashMap();
 
   /**
    * Creates a [LinkedHashMap] that contains all key value pairs of [other].
    */
   LinkedHashMap.from(Map<K, V> other);
 }