Update SplayTreeMap with a work-around for frog.

corelib/src/implementation/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.
 
 /**
  * A node in a splay tree. It holds the key, the value and the left
  * and right children in the tree.
  */
 class SplayTreeNode<K, V> {
   SplayTreeNode(K this.key, V this.value);
@@ skipping 256 matching lines @@
     splay_(node.key);
     return node.key;
   }
 
   /**
    * Get the last key in the map that is strictly smaller than [key]. Returns
    * [null] if no key was not found.
    */
   K lastKeyBefore(K key) {
     splay_(key);
-    K visit(SplayTreeNode node, [K ifEmpty]) {
+    K visit(SplayTreeNode node, K ifEmpty) {
       if (node === null) return ifEmpty;
       if (node.key.compareTo(key) >= 0) {
         return visit(node.left, ifEmpty);
       }
       if (node.key.compareTo(key) < 0) {
         return visit(node.right, node.key);
       }
     }
-    return visit(_root);
+    return visit(_root, null);
   }
 
   /**
    * Get the first key in the map that is strictly larger than [key]. Returns
    * [null] if no key was not found.
    */
   K firstKeyAfter(K key) {
     splay_(key);
-    K visit(SplayTreeNode node, [K ifEmpty]) {
+    K visit(SplayTreeNode node, K ifEmpty) {
       if (node === null) return ifEmpty;
       if (node.key.compareTo(key) > 0) {
         return visit(node.left, node.key);
       }
       if (node.key.compareTo(key) <= 0) {
         return visit(node.right, ifEmpty);
       }
     }
-    return visit(_root);
+    return visit(_root, null);
   }
 }