Fix inconsistent APIs

lib/src/unmodifiable_wrappers.dart

 // Copyright (c) 2013, 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.
 
 /**
  * Wrappers that prevent a List, Set, or Map object from being modified.
  *
  * The [Set] and [Map] wrappers allow reading from the wrapped collection,
  * but prohibit writing.
  *
@@ skipping 209 matching lines @@
   /**
    * Throws an [UnsupportedError];
    * operations that change the map are disallowed.
    */
   void addAll(Map<K, V> other) => _throw();
 
   /**
    * Throws an [UnsupportedError];
    * operations that change the map are disallowed.
    */
-  V remove(K key) => _throw();
+  V remove(Object key) => _throw();
 
   /**
    * Throws an [UnsupportedError];
    * operations that change the map are disallowed.
    */
   void clear() => _throw();
 }