Fix some cases where Map.containsValue did not accept Object as argument.

sdk/lib/collection/splay_tree.dart

 // Copyright (c) 2012, 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.
 
 part of dart.collection;
 
 typedef bool _Predicate<T>(T value);
 
 /**
  * A node in a splay tree. It holds the sorting key and the left
@@ skipping 307 matching lines @@
     SplayTreeMap<K, V> map = new SplayTreeMap<K, V>(compare, isValidKey);
     Maps._fillMapWithIterables(map, keys, values);
     return map;
   }
 
   int _compare(K key1, K key2) => _comparator(key1, key2);
 
   SplayTreeMap._internal();
 
   V operator [](Object key) {
-    if (key == null) throw new ArgumentError(key);
     if (!_validKey(key)) return null;
     if (_root != null) {
       int comp = _splay(key);
       if (comp == 0) {
         _SplayTreeMapNode mapRoot = _root;
         return mapRoot.value;
       }
     }
     return null;
   }
@@ skipping 490 matching lines @@
     return new _SplayTreeNode<E>(node.key)..left = _copyNode(node.left)
                                           ..right = _copyNode(node.right);
   }
 
   void clear() { _clear(); }
 
   Set<E> toSet() => _clone();
 
   String toString() => IterableBase.iterableToFullString(this, '{', '}');
 }