Use relative URIs for core library part-of statements.

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;
+part of "collection.dart";
 
 typedef bool _Predicate<T>(T value);
 
 /**
  * A node in a splay tree. It holds the sorting key and the left
  * and right children in the tree.
  */
 class _SplayTreeNode<K> {
   final K key;
   _SplayTreeNode<K> left;
@@ skipping 846 matching lines @@
   }
 
   void clear() {
     _clear();
   }
 
   Set<E> toSet() => _clone();
 
   String toString() => IterableBase.iterableToFullString(this, '{', '}');
 }