Move SplayTreeMap from coreimpl to collection.

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.
 
 /**
  * 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 286 matching lines @@
       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, null);
   }
 }