Use typedefs in lib.

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;
 
 /**
  * A node in a splay tree. It holds the sorting key and the left
  * and right children in the tree.
  */
@@ skipping 223 matching lines @@
  *
  * The map is based on a self-balancing binary tree. It allows most operations
  * in amortized logarithmic time.
  *
  * Keys of the map are compared using the `compare` function passed in
  * the constructor. If that is omitted, the objects are assumed to be
  * [Comparable], and are compared using their [Comparable.compareTo]
  * method. This also means that `null` is *not* allowed as a key.
  */
 class SplayTreeMap<K, V> extends _SplayTree<K> implements Map<K, V> {
-  // TODO(ngeoffray): Restore type when feature is implemented in dart2js
-  // checked mode. http://dartbug.com/7733
-  Function /* Comparator<K> */_comparator;
+  Comparator<K> _comparator;
 
   SplayTreeMap([int compare(K key1, K key2)])
       : _comparator = (compare == null) ? Comparable.compare : compare;
 
   /**
    * Creates a [SplayTreeMap] that contains all key value pairs of [other].
    */
   factory SplayTreeMap.from(Map<K, V> other, [int compare(K key1, K key2)]) =>
       new SplayTreeMap(compare)..addAll(other);
 
@@ skipping 321 matching lines @@
 class _SplayTreeValueIterator<K, V> extends _SplayTreeIterator<V> {
   _SplayTreeValueIterator(SplayTreeMap<K, V> map): super(map);
   V _getValue(_SplayTreeMapNode node) => node.value;
 }
 
 class _SplayTreeNodeIterator<K>
     extends _SplayTreeIterator<_SplayTreeNode<K>> {
   _SplayTreeNodeIterator(_SplayTree<K> map): super(map);
   _SplayTreeNode<K> _getValue(_SplayTreeNode node) => node;
 }